Robert Lang's Galaxy Letters 1-38

(1)

Title: NGC 4388 - A Weird Wonder of The Sky

Dear fellow galaxy fiends,

Here's an image to to interest even the most cosmically jaded - the edge-on
Seyfert Galaxy NGC 4388 in the Virgo Cluster of galaxies. (
N4388_(subaru)_display.jpg - image file attached)
The other image file is a negative, to show the highly peculiar dust lanes
- see later in this email for info.

The pink and red colour-coded material in the image is ionized gas, i.e.
emission nebulosity. This was detected in two exposures with the 8m class
Subaru telescope, which were, respectively, taken at the two specific
wavelengths in which the warm ionized gas of a nebula emits profusely - the
emission lines known as OIII, and Hydrogen Alpha. The two emission line
images were added to a Subaru V-band [ roughly visual wavelengths ] image to
produce the image in the attachment. (see...www.subarutelescope.org for
higher resolution version of the image )

Note the large ionized gas cone or plume! I am not sure how much of this gas
was ejected by the activity in the nuclear region. Conical outflows of gas
originating from the active
nuclear regions of galaxies are not particularly rare, but they generally
lack the power to push the gas very far out of the disk.
(except in the most extreme objects such as NGC 1275 in Perseus where the
central star formation and/or the Active Galactic Nucleus are powerful
enough to effect the environment of the galaxy, even 100,000 light years
away from it!)

In fact, very faint H-alpha emitting ionized gas (= nebulosity) is not that
rare, seen outside of the bright portions of the disks of spiral galaxies. Some
examples - NGC 253, NGC4631(my favourite northern galaxy), NGC 5775, NGC
2188, NGC 891.

The commonality between the above listed galaxies is that the planar disk
component has a very high rate of formation of massive stars, well
elevated over the star formation rate in the disk of the Milky Way.

Faint H-alpha/OIII emitting gas when seen far above or below the principal
plane of a spiral, has two typical sources:

(1) It may be ejected out of, and at right angles to, the
principal plane of a galaxy by a central source of energy. This source is
usually in a compact area near the the nucleus of a galaxy - these central
sources are often powered partly by an active Seyfert nucleus and partly by
a surrounding ring of violent star formation(= "circumnuclear starburst").
These gas outflows tend to be conical in shape.

(in a few galaxies (e.g.NGC 1365), there is noticeably more star formation
going on in the very central region than in the rest of the galaxy put
together! A powerful circumnuclear burst of massive star formation)

(2) Extraplanar ionized gas can be
ejected far out of the planar disks of spiral galaxies by the effects on the
interstellar medium of heating and shock waves from multiple supernovae.
Thus - extraplanar ionized gas is reasonably prominent in galaxies which have
a high global star-forming rate in the disk, and which thus have large
numbers of supergiant stars in the disk.

There are several other interesting things in this image of NGC 4388, and
these are
things which may be visually more obvious than the faint H-alpha plumes
seen in the image:

(a) There is a very obvious asymmetry between the eastern and western halves
of this galaxy. The faint outer disk, with its smooth spiral arms, is a
different shape on the two sides of NGC 4388 - as we go out along the
major(long) axis of this galaxy, starting from the central bulge region,
there is a large difference in shape between the two sides of this galaxy!
The bright inner disk region is also asymmetric.
This may well be a genuine case where the mass distribution of a galaxy is
distorted. An interesting comparison is NGC 4631, which also has a
distorted disk.

(b) The heavy dust lane on one side, and the enormous star forming complexes
here, have no similar counterpart on the other side of the galaxy. Further,
the dust lane may twist out from the galactic plane further than is
usual in undisturbed spirals. The firehose-like twisting of the dust lane
may be
anomalous, in this case ! The "snake-like" dust lane is very well shown in
the image
N4388_twistydustlane.jpg, which is a negative of the main body of the
galaxy, converted to
gray scale.

Sincerely, vmadbadgalaxyman

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Title: Ara to Circinus - The Unknown Milky Way

Dear galaxy friends,
This is further to my previous comments on the lack of observations of
southern nebulae in the catalogues of Gum and of Rodgers Campbell and
Whiteoak(the RCW catalogue). My previous comments were -

" (1)Some star atlases plot hardly any HII regions in a vast
area of Milky Way, between Dec -44 and Dec - 63 and between Galactic
Longitude 342 degrees and 301 degrees. (essentially the Milky Way between
southernmost Scorpius and the Southern Cross).
(2) There seems to be little information as to which of these HII regions
are worth observing.
(3) This is a sort of "Terra Incognita", even for southern observers,
which
is odd...given that we seem to know the HII regions in Scorpius and
Carina
rather well. "

I might add to the above, that what is really needed is a catalogue of
"observable" southern HII regions, for input into the various star atlases,
rather than the existing input catalogues, which contain objects requiring
a two hour H-alpha exposure.

The complete set of papers covering the relatively shallow H-alpha survey
of Y.M.Georgelin and colleagues, which is ideal for identifying prominent
southern HII regions, is as follows:

Astronomy and Astrophysics , (1992), 257, 389
(Deals mainly with RCW 70, a small nebula in the same line-of-sight as the
Coalsack. )

Astronomy and Astrophysics Supplement (1994), 108, 513
( HII regions in the galactic longitude 328 degrees region of Norma)

Astronomy and Astrophysics Supplement, (1996) , 120, 41
(HII regions in Norma, Ara, and Southernmost Scorpius milky way)

Astronomy and Astropysics Supplem nt, (1998), 130, 119
(HII regions of the Circinus, Centaurus and Crux milky way )

Astronomy and Astrophysics, 357, 308 (2000)
( Covers about 5 by 5 degrees of the Carina Milky Way, starting to the East
of the Eta Carinae nebula, and then going further East . Beware! In this
paper, their H-alpha images are split, in such a way that each "image"
displays only the nebulosity at a particular distance! )

A good way to find these nebulae is to set your electronic star atlas to
display galactic longitude and galactic latitude, as each paper gives
mainly the galactic coordinates of each HII region. Alternately, the
H-alpha images in the papers can be used as finder charts. The software,
"Guide V.8" can also be set so as to display RCW objects. (I am unsure
about Megastar)

(galactic coords are simply defined by angular distance along the apparent
plane of the Milky Way and by angular distance from the plane at right
angles to it. )

In particular, I note that several of these RCW objects were plainly
visible in the 14 inch telescope of Peter Marples, even in relatively poor
conditions - but have an OIII or other nebula filter handy when you view
them. The brightest of these objects ought to have made the cut into the
NGC/IC !

(I have previously mentioned the spectacular RCW108, the "Ghost of Eta
Carinae" nebula, which extends for 1-2 degrees from the cluster NGC6193. )

I also note that the AAO/UK Schmidt H-alpha survey of the southern milky
way is producing many pretty images - but this is an exceedingly deep
survey, and most of these HII regions will be V.V.V.V.V.Faint.

Regards,
Robert Lang

P.S. There is a new and much-needed master catalogue of dark nebulae,
which subsumes all previous catalogues -
Dutra and Bica, Astronomy and Astrophysics, (2002), 383, 631
( the catalogue proper should be available at http://cdsweb.u-strasbg.fr)

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Title: Carina Nebulae - what next after Eta Car?

Dear Mr Hydrogen-alpha ,

We recently viewed IC 2948 with a good 14 inch - with averted vision, the
nebula was extremely obvious and indeed well structured, covering a large
telescopic field - however, it takes a lot of straining to see it really
well with an 8 inch, and it is rather structureless with this smaller
aperture. However, the integrated brightness is very high, and it is a
bright patch of pink on any shot of the Milky Way.

The several main bright condensations of the N3576-79-81-86 nebula complex
were blatant and well-structured with a 14 inch (no imagination required),
and literally appeared almost photographic in brightness with a 16. Here
again, they can be picked up in an 8 inch, but the structure is very
difficult to discern. These nebs are apparently in the Carina spiral arm
(as is Eta Car).

I have known these two nebs for a long time, but I admit I have never seen
or noticed NGC 3572, which is near to the N3576 complex and to NGC 3603
(cluster plus neb.) and to NGC 3532 (the "Football Cluster").

Cheers, Robert Lang, galactophile

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Title: NGC 3603 - Between a globular and an open cluster

Fellow galactic trekkers

The Young Massive Compact Cluster (also can be termed a "super star
cluster") called NGC 3603 was definitely resolved, at least in its outer
regions, when it was viewed with Peter Marples' excellent 14 inch
Newtonian telescope.

The cluster core is only 30 arcseconds across, with extensions to a
diameter of 1 arcminute, so this is a challenging object! The surrounding
nebula is a giant HII region, though less luminous than the Eta Carinae
Nebula.

NGC 3603 sits BEHIND the Carina spiral arm, in the NEXT spiral arm outwards
along our line-of-sight - so there is substantial foreground extinction as
a result - it is at a distance of some 25000 light years, and there is 4-5
magnitudes of visual extinction in front of it.

NGC 3603 is only 1-3 million years old, and is similar to a low-mass
newly-formed globular star cluster.
As a result of its youth, N3603 contains the full range of known stellar
masses, from 120 solar-mass supergiant stars through to small subsolar-mass
stars.

(( note for the uninitiated - our own Galaxy's globulars are all at least
9 billion years old, with some evidence found recently for an age range of
3 billion years in the cluster population. In contrast, a few intermediate
age (~ 3-5 billion years old ) globular clusters are known in the peculiar
southern candidate merger system NGC1316, which is an elliptical/S0 galaxy. ))

Young massive compact clusters like NGC 3603 .....generally have
globular-cluster-like stellar distributions (technically, a King Profile) ,
and the prototypical objects, R136 (in Tarantula Nebula) and NGC 3603, have
masses of 10000-40000 solar masses - perhaps a little low to be regarded as
bona fide "young globular clusters", especially when taking into account
gradual loss of stars over the ensuing billions of years.

Howevever, violently starforming galaxies like M83 and NGC 1313 contain
some newly formed clusters which have masses of some 100, 000 solar masses
- well within the typical range of masses of our own Galaxy's exclusively
old globular star clusters. Given enough gas, and the right conditions, it
seems that a giant HII region and its associated giant molecular cloud can
actually form a young globular star cluster. For instance, the
approximately 100 known young globular star cluster candidates in the
galaxy M83 are 16th to 19th magnitude visually.

My favoured interpretation of the facts is that, with increasing total mass
at their formation, star clusters become increasingly
"globular-cluster-like" and decreasingly "open-cluster-like". Thus, open
star clusters and globular clusters form a continuum, with the difference
in structure being due mainly to the differing mass.

For my newer correspondents, I reiterate that the southern dwarf galaxy NGC
1705 also contains a bright "young globular star cluster candidate object."

regards, from the vmadbadgalaxyman

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Title : NGC 3603, and its relation to young globulars

I have mentioned NGC 3603 as being a candidate for either a young globular
star cluster, or (much more likely) a transition object between a massive
open star cluster and a low-mass globular star cluster.

It was shown recently that the stellar density profile of the N3603 cluster
is identical to that of a globular cluster - but this cluster is probably a
little too low in mass to be a newly formed globular star cluster.

Interestingly, galaxies with many supergiant HII regions seem to have the
ability to produce young clusters which are several times as massive as
NGC3603, and which very likely do qualify as bona fide young (newly formed)
globular star clusters. A good example of this is M83, in which some of its
HII regions have been shown to host young clusters of at least 100,000
solar masses involved with them.

The moral of the story seems to be that if you have a large enough mass of
gas, it is possible to produce new clusters which are of similar mass to
globulars. It would seem that the HII regions and their attendant molecular
gas complexes in our own Galaxy.... are not usually massive enough to
produce these clusters. (however, the 2 micron all-sky survey has found a
handful of possibles in the central region of our galaxy)

Cheers, Robert Lang

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Title: When a peculiar galaxy is not actually peculiar

There is no doubt that people associated with the amateur side of astronomy
have developed tools which can save professional astronomers a lot of time.
For instance, I now know of a few professionals who use "Guide" because it
saves them many time-consuming accesses to NED and LEDA.
As another example , I was able to use this atlas to find some
supergalactic structures which were of interest to a couple of
professionals - it was simply a matter of finding patterns in the apparent
2-D distribution of galaxies. (actually, the ability to scan a lot of
galaxies and click on them quickly to find their redshifts can be very
useful - for instance, I have found some subcluster structure in the Antlia
Cluster and the Dorado Group, simply by noticing that the galaxies were
grouped in redshift space - I will probably write a news note on this for
my mailing list. )

I hit peculiar galaxies, or should I say "peculiars", regularly in my
astronomical work/play. The problem with many apparent morphological
"peculiarities" is that they are not necessarily real peculiarities. For
example;

- one strong arm and one weak arm seems to be intrinsic to the idea of a
barred spiral galaxy. (indeed, only one arm is almost the norm for types
later than Sd.)

- Barred spirals are generally subject to a bewildering variety of
asymmetries.

- The Hubble sequence is defined by blue images - and blue images
emphasize HII regions, OB stars, and dust, which are very minor mass
components in galaxies. In contrast, even some rather bizarre galaxies can
look quite regular in the near-infrared.
( Galaxies like NGC 300 or NGC4395 have a nearly random distribution of
HII regions, but not so the mass distribution of the galaxy. )

- Some asymmetries apparent on inspection.... are not actually so - it is
easily possible to convince oneself of an asymmetry when a galaxy is not in
fact very asymmetric. In a number of cases, when I had thought a galaxy was
weird, it turned out to be only slightly unusual when I actually measured
it, particularly in images at >=800 nm wavelength.

- Some type Sd - Sm galaxies look like they may have undergone a merger or
interaction, but they have not. For instance, NGC 1313 looks pretty weird
at first sight, but it turns out to be just a normally rotating disk galaxy.

- It is difficult to get a handle on what is the real 3D shape of a galaxy,
which makes it difficult to get a mental picture of its true structure.
This difficulty is compounded by factors such as.... the occasional
presence of several tilted disks within a single normal spiral......the
fact that many spirals are intrinsically oval at the level of b/a =
0.94......the routine presence of several structures of even higher
intrinsic ellipticity than this within a single barred spiral galaxy....the
presence of some ellipticals which are oblate spheroids (hamburger bun
shaped) and the presence of other ellipticals which are prolate spheroids (
hot dog bun shaped)......etc.... etc...

In my view, NIR data is a big help in sorting the mildly peculiar from the
very peculiar. Unusual Kinematic data can also provide a strong indication
of the existence of peculiar interactions mergers or accretion in a
galaxy. Strongly elevated star formation rate may be a sign of interaction
between galaxies, but can have other causes, such as inflow to the
circumnuclear region of massive quantities of gas along a bar.

Generally, I also look for signs of a very high central mass of molecular
gas in a galaxy, if trying to confirm its "peculiarity" - though here
again, the origin could either be from an interaction and/or from the
effects of a bar.

A very high far infrared luminosity in a galaxy is generally a strong
indication that some sort of interaction has taken place (or is taking
place) - If a galaxy has an FIR luminosity of at least 100 billion solar
luminosities, it is a safe bet that something very drastic has happened to
its interstellar medium.
In my experience, the majority of galaxies where the FIR luminosity of the
system is 10E11 or 10E12 solar luminosities..... are generally the subject
of violent star formation bursts due to interactions with other galaxies.
(only a minority of these interacting/colliding systems have this enormous
FIR luminosity being produced by a central active galactic nucleus)

cheers , Robert Lang
galaxyco@tpg.com.au

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Title: NGC 3269 - smooth arm Sa spiral - possibly stripped !

Dear colleagues,
Re: NGC 3269 and the Antlia cluster of Galaxies
NGC 3269 is a smooth-arm spiral. which is a common type in clusters of
galaxies , but relatively rare in the low density regions of the "galaxy
field". (see two attached images of it)

It is in the Antlia cluster core surrounding NGC 3268, which is an
interesting but challenging target for both imaging and visual observation
- the Antlia cluster is the 4th nearest major cluster, after Virgo, Fornax,
and the very obscured cluster in Puppis. As such, it is easier to observe
than the Hydra Cluster or Centaurus cluster, which are somewhat more
distant. (see Antlia2.jpg for a chart of the cluster core)

Here are two versions of an image of NGC 3269 in the Antlia Cluster of
galaxies. As you will read below, N3269 may be thought of as an "anemic"
galaxy with very smooth arms and which is very gas poor. (however, the
proposer of the Anemic classification, S. van den Bergh, disagrees with me!)

Below, I put forward the provocative idea that, because it has a relative
velocity of 1000 km/s in our line-of-sight.... with respect to the core of
the Antlia Cluster as defined by the giant elliptical galaxy NGC3268 ( yet
it appears to be at the same distance) - that NGC 3269 is part of a group
of three galaxies passing through the cluster core (or which have already
passed through the cluster core).

Further, the passage through the cluster core may have actually stripped
N3269 of most of its gas, leading to the smooth arms with little star
formation! Cluster cores contain large masses of very hot (10 or more
million degrees) gas, albeit at very low density - models indicate that
pressure from this gas can push out a lot of the gas contained in a galaxy
as the galaxy passes through the cluster core.

One theory for the origin of S0 (lenticular) type galaxies is that they are
basically spirals which have been stripped of their gas. (it is also
possible that some S0 galaxies lost their gas, very early in the history of
the universe).

Q. Is NGC 3269 becoming an S0 galaxy?

Note; the below non-extinction corrected distance modulus of 32.73
corresponds to a distance of only 104 million light years for the Antlia
cluster. (compare to 50-60 million light years for the Virgo cluster)
Others prefer distances closer to 130-140 million light years for Antlia
cluster. I suspect that there exists more recent data on the distance of
this cluster.
Cheers, Robert Lang, Mad bad and dangerous Amateur Astronomer

>To: Sidney van den Bergh
>From: Robert Lang
>Dear Dr van den Bergh
>The below emails, and also the attached images, relate to a very bright
>(B = 13 and D25 = 2.5') southern anemic system - NGC3269

>>To: Anthony_Fairall From: Robert Lang
>>Subject: NGC3269
>>Dear Tony Fairall
>>The system mentioned in the below email is a very interesting galaxy and
>>a very interesting situation!
>>Two subclusters of the Antlia cluster are known, from the apparent
>>distribution of galaxies in the sky, and this was also indicated by
>>X-ray observations (see ref below) - one is centred on N3268 and
>>the other is centred on N3258.
>>However it seems obvious from the presence of a group of redshifts which
>>are, apparently, uniformly 1000km/s higher than those of the two
>>ellipticals I just mentioned, that there could be a third subcluster
>>which is separated in velocity space, though perhaps not in 3D space.
>>Robert Lang, galaxyco@tpg.com.au
>>
>>>To: Ken Freeman From: Robert Lang
>>>Subject: NGC3269
>>>Dear Ken
>>>"Just for fun" ...... I attach two versions of a really interesting
>>>image of NGC 3269, the bright "smooth-arm very early Sa" (to use the
>>>Hubble-Sandage classification) or "anemic" type (to use van den Bergh's
>>>classification) in the Antlia cluster(s).
>>>
>>>It was taken by the MOSAIC camera mounted on the 4-metre CTIO telescope
>>>by Dirsch et al, (2003, A&A, 408, 929) - actually, they were studying
>>>NGC3268 and NGC3258 and their GCs - the two big ellipticals in the
>>>Antlia Cluster. ( I believe that they have also recently obtained VLT
>>>imagery of N3268 to beyond the turnover of its GCLF) .
>>>
>>>A good recent summary on the Antlia cluster can be found in A&A, 408,
>>>929 (2003)
>>>
>>>Dirsch et al. favour m-M = 32.73 for the Antlia cluster, averaged
>>>from two individual SBF galaxy distances and two Fundamental Plane
>>>galaxy distances. I recall that Sandage favoured a much larger distance
>>>in his pioneering study of this cluster: m-M = 34.1 (Ferguson and
>>>Sandage, 1990, AJ, 100, 1)

>>>Apart from the interesting morphology of NGC3269, it appears to be one
>>>of three prominent early-type disk galaxies which are (all three of
>>>them) at a redshift 1000 km/s higher than the cluster! (the other two
>>>galaxies which are 1000 km/s higher are N3271 and N3267)
>>>
>>>The three redshift-discrepant galaxies APPEAR TO BE part of a
>>>"normal-looking dense cluster core" surrounding the giant elliptical NGC
>>>3268, yet their velocities are (probably) uniformly 1000km/s higher -
>>>this could possibly be a situation where high orbital velocities, or an
>>>infalling group, are present . (e.g. similar to the situation in the
>>>Centaurus Cluster, where the Cen 45 group of galaxies appears to be
>>>falling into the main cluster)
>>>
>>>So, if NGC 3269 and the other two higher velocity galaxies do form a
>>>subgroup and they are either falling into the Antlia cluster, and/or
>>>passing through the core of the Antlia cluster, this galaxy could be a
>>>really excellent test-case for the idea of gas stripping in galaxies by
>>>the effects of the ICM. (and much easier to observe than in more
>>>distant clusters)
>>>
>>>The case for the possible gas-stripping of NGC 3269 (or others will say
>>>that the ISM of N3269 was used up earlier in the evolution of the
>>>galaxy) is strengthened because it is apparently greatly deficient in
>>>gas, and because of the substantial hot ICM in the Antlia cluster.
>>>
>>> Koopmann and Kenney have been on the trail of gas-stripped galaxies
>>> for a while (e.g. AJ, 117, 181 - on NGC 4522)

>>>The distance modulus adopted by Dirsch et al would be consistent with a
>>>distance of about 2600 km/s for the Antlia cluster, for Ho = 75 .
>>> Cheers, Robert Lang, Amateur Astronomer

(8)

Title: Massive, young, compact and concentrated = young globulars?

I have written a lot regarding young massive compact clusters such as
NGC3603, the Arches and Quintuplet clusters (obscured objects near the
galactic centre) , R136 (the core of the 30 Doradus Star Cluster), and also
about even more massive young star clusters such as the ones in NGC 1705
and M82, and those surrounding the nuclei of NGC 1365 and NGC1097.

N3603 was recently shown to have a "King Profile", which indicates that it
is extremely centrally concentrated and regular. Nevertheless. NGC 3603 may
not quite "make it" as a newly formed globular.

N3603 is probably no more than about 10000 solar masses, while R136 may be
as massive as 50000 solar masses - thus, R136 is probably massive enough to
be called a young globular star cluster.

It would seem that there is a continuum between "massive compact clusters"
such as these....and young globular clusters of the sort that are known in
galaxy merger systems such as NGC4038/NGC4039, NGC 7252, and NGC 3256. It
may simply be that as clusters get more and more massive, they get less
"open cluster like" and more "globular cluster like"

Regards, Robert Lang
galaxyco@tpg.com.au

(9)

Title: DDO154 and NGC2915 - do dark galaxies exist?

Dear galaxy people,

Dr Kenneth C. Freeman notes, in ASP Conference Series Vol. 275 (2002) p.3
, (the meeting was on the topic of the disks of galaxies), that examples
exist of apparently "unevolved galaxies" :
- which are overwhelmingly made up of dark matter
- which have a a significant minority of the galaxy's total mass in gaseous
form
- which contain the merest smattering of stars, as compared to the enormous
masses of gas and dark matter found in the system.

The two examples of extreme "dark matter and gas dominated" galaxies which
he gives in this symposium are DDO 154 (= PGC 43869) and NGC 2915

In these two systems, the ratio of dark-matter-mass to gas-mass to mass
which is found in stars
is......... 100 : 10 : 1 ( !!!!!!!!!!!!!!! )

One would, naively, expect that the large gas mass in these galaxies would
eventually form a more substantial stellar component - given enough time.

If memory serves me correctly, DDO 154 is particularly elusive - the RC3
gives a mean surface brightness about 2 magnitudes down on the value which
is typical in large galaxies.

The central region of NGC 2915 is of more normal optical surface
brightness, but NGC2915 has a huge and massive halo of neutral hydrogen gas
. (extending to 5 times the Holmberg radius)

As Dr Freeman is one of the least recognized of our great Australians, I am
sure he will appreciate the free publicity ! He is similar to the likes of
Alan Sandage or Fritz Zwicky, in that he has made very major contributions
to virtually every subfield of astronomy. (if Freeman was as good at
cricket as he is at astronomy, he would be a household name in Australia! )

If you have followed the recent astronomical news, you will have heard the
mutterings about the discovery of "dark galaxies" i.e. galaxies which are
composed ONLY of dark matter.

Given that there is more dark matter than stellar matter in normal
galaxies, this is not necessarily a surprising discovery.

( of course if the standard law of gravity is modified at large
galactocentric distances, then the dark matter does not exist, as in the
theory of MOND (modified newtonian dynamics) )

Robert Lang, A.A.E.F. , R.B.C.A.
(Amateur Astronomer of Extreme Fanaticism)(Rob's Backyard Centre For
Astrophysics)

(10)

Title: IC 4662 - A Galaxy with two supergiant HII regions

IC 4662 is a bright and obvious dwarf irregular galaxy in Pavo, which is
currently undergoing a violent burst of star formation. It contains two
very easily observable extragalactic supergiant HII regions.
IC 4662 was studied by Heydari-Malayeri et al., in A&A, 234, 99 (1990) .
As Professor Ken Freeman notes below, HII regions on the scale of The
Tarantula, or of the two big nebulae in IC 4662 are often the host of young
massive blue clusters of similar size and mass to normal old globular
clusters. (i.e. candidate young globulars)
For instance, the core of the 30 Doradus star cluster, known as R136, may
be regarded as similar to a low mass globular cluster, but at a young stage
of evolution.

At 04:06 PM, Ken Freeman wrote: (re Super Star Clusters = young globular
candidates)
........ A really large HII region is often the site of an SSC...... ( =
young massive compact cluster)
Cheers , Ken

(11)

Title: NGC 3256 - a merger of two galaxies

Dear Galaxy nut,

A good southern galaxy-merger system, which is further along in evolution
than NGC 4038/9 is NGC 3256. (i.e. in NGC 3256 the progenitor galaxies
have already merged)
NGC 3256 contains 1000 newly-formed globulars, plus 5 nebulae which are
each 85 times as luminous as the Tarantula!

N3256 is not an ultraluminous infrared galaxy (i.e. over 10E12 solar
luminosities in the infrared), but almost! Due to a very high star
formation rate, its time-rate of core-collapse supernovae is calculated to
be very high indeed, but it may require NIR observation or the use of
longer CCD exposures to detect its supernovae. (severe internal dust
extinction exists)

The morphology of N3256 is as obvious visually as is the morphology of
N4038/9 - even a 10 inch once showed NGC3256 as obviously a merger system.

Thus, the star formation rate is extraordinarily high. Its many (about 1000
known) SSCs were recently studied by Ken Freeman and Jayanne English and
colleagues - see Astronomical Journal, 125, 1124 (2003). There is also a
previous paper by them on the SSCs in NGC3256.

See also their HI (neutral hydrogen, kinematics, dynamics, and evolutionary
history) paper on this galaxy - AJ, 125, 1134 (2003)

They note that NGC3256 includes 5 HII complexes, each having a luminosity
comparable to 85 times that of the Tarantula Nebula !! (now that's what I
call a star formation burst! )

Another system recently mentioned as about to undergo a merger of two
galaxies is NGC5427 and NGC 5426.

I have no further information on N5426/5427, but hadn't realized this was
a candidate member of the "merger sequence" in time (as was first properly
modelled by Toomre) - the Carnegie Atlas of Galaxies referred to this pair
as being morphologically very little disturbed.

As far as Super Star Clusters go, I do prefer the alternative descriptor
- young massive compact clusters ( = YMCCs or MCCs)
(which better captures their status as young objects occupying the same
mass and radius range as globular clusters) .
However....Freeman and English call themYoung Populous Clusters !!
(YPCs) There is obviously no agreed name for these amazing objects, as
others prefer to call them Young Globular Candidates....(YGCs)

Robert

(12)

Title: NGC 7582, NGC 1097, N613, N1672, N1313

Dear astro friends,

The bright nuclear region of NGC7582 actually contains a very tight but
obscured ring of supergiant starforming regions and innumerable
supermassive very-hot stars. These energize a super-duper-large and
ultraluminous central H-alpha emitting nebula which is plainly visible on
amateur images.

When we take into account the fact that there are, at the very minimum,
several magnitudes of dust extinction in front of the bright nuclear
region of N7582, coming from the very dusty "flat disk" component of this
galaxy, the energy output of the bright nuclear region would actually
dominate the view of this galaxy, were it possible to obtain a view without
the heavy dust obscuration within this galaxy. ( this is also the case for
NGC 253 !)

A nice, relatively unobscured, and particularly large circumnuclear burst
of star formation is at the centre of N1097. (another of my
favourites) The ultra-giant star-forming- regions form a patchy ring which
is rather large ( I think it could be 15'' across). This ring should be
amenable to visual observation.

Generally, when you take a 2.2 micron (near infrared) image of one of these
"critters" which has a violent central starburst, the nuclear region
appears extremely prominent indeed - due to the fact that dust extinction
from the dust screen within a galaxy is only 1/10 the extinction in a
yellow light image when that galaxy is imaged at the near-infrared
wavelength. of 2.2 micrometres.

Those special NIR CCDs (necessary for imaging at wavelengths longer than 1
micron = 10000angstroms) have got very good of late (commonly up to 1024 by
1024 pix). However, I don't know how much one of these things would cost.
As you know, I have been trying to point out the advantages of
very-near-infrared imaging at 800nm to 1000nm(1micron) using standard CCDs,
for some time...especially where you have a highly obscured object .

Do you know the barred spiral NGC 613? It has one of the most interesting
morphologies of the "normal" galaxy population. It is about 5 arcminutes
across, so is a good target for imaging.
N613, N1672, and N1808 are probably my current favourite southern barred
spirals, as they all have unusual morphological features and
irregularities. In particular, N613 and N1672 are not often imaged by
anybody or studied by anybody.

I also "have a crush on" the little-studied NGC 1313, which is a very
large (8 arcmin diameter) transition object between barred irregular
galaxies and barred spirals. (there have only been one or two papers on
this galaxy, in all the history of astronomy, because it is far southern)
NGC 1313 is also one of those few galaxies where the very detailed
morphology is "plain as day" when visually observed with a 10 inch
telescope. ( NGC 613 has had even less attention!! )
N1313 does have spiral arms and a bar, but only in incipient form. It looks
very raggedy in images and also visually, but this is only because it is
about half-way to being an irregular - its MASS distribution is actually
extremely regular, as revealed by observations of its HI and of its old
stellar population.
(same can be said the LMC, which carries the barred disk galaxy morphology
even further along the Hubble Sequence in the direction of true irregular
galaxies ! )
Large starforming patches can be seen all over a large field when N1313 is
observed visually.

Cheers, Robert Lang

(13)

Title: N4038&4039 - can you see its young globulars?

You may be interested in this summary of correspondence between myself and
Rich Jakiel regarding the visibility of the young globular clusters in the
NGC4038/NGC4039 system, which were formed 6-10 million years ago in the
collision of these two galaxies. These types of clusters can reach 5
magnitudes (intensity factor of 100x) more luminous than our own Galaxy's
most luminous globular cluster, Omega Centauri.

Summary of Correspondence:

(1) Robert Lang wrote: (re: young globular star clusters in the Antennae
galaxies)
Mass estimates for the brightest clusters are all in the range 640,000 to
4.7 million solar masses. Similar, in fact, to the masses of the largest
globular star clusters in M31, M81, and the Milky Way.
I don't think that there can be much doubt that these objects in the
Antennae galaxies are newly formed globulars. (they are some 8-10 million
years old)
(2) Rich Jakiel wrote:
What is the apparent mags on these objects? About 1 1/2 yrs ago, I was
using a 20-inch starmaster under superb conditions (at close to 600x) and
noticed that tiny 'stars' of ~ 17mag where visible inside some of the
knots/condensations.
(3) Robert Lang wrote:
Your observation sounds most plausible, at first impression.
I should be able to work out how bright these globulars are
(4) Robert Lang wrote:
I haven't had time to check my working, but Whitmore et al (1999,
AJ,118,1551) measured the apparent mags of the young clusters in
N4038/N4039. The brightest cluster had V absolute magnitude -13.92, for a
distance modulus of m-M = 31.41. This should translate to this most
luminous cluster having an apparent V magnitude of 17.5.
(the assumed distance of N4038/9 is 19.1 Megaparsecs = 62 million light years)

Their 3 most luminous clusters were in the range: absolute magnitude -13 .7
to -13.9, which translates to a V apparent magnitude of 17.5 to 17.7.

Note, however, the rest of the clusters were V apparent magnitude 18.4 or
fainter.

I think that you could probably glimpse a 17th magnitude object with a 20
inch, so it is likely that the young overluminous globulars are mostly
somewhat too faint for your magnitude limit, especially when you consider
that there are only three of them which are as bright as V = 17.5 to 17.7

This is, perhaps, a slightly disappointing result, however......

Your description of the stellar-like objects that you viewed within the
knots in N4038/9 probably matches that of some more extended (and more
luminous) stellar associations in this system, and some of these could
EASILY be 16-17 apparent magnitude. (for instance the Tarantula nebula
complex would probably be roughly 17th magnitude at the distance of N4038,
and even larger examples of this sort of thing exist in other galaxies.)

What I do see on the N4038/9 images from the HST is some bright "cores" of
the HII regions, which are more akin to the cores of the Tarantula Nebula
or NGC 604 (in M33) complexes - they appear slightly more extended than the
young blue globulars, and indeed may INCLUDE a young globular. These
objects look a lot brighter than the globulars, and may be more akin to an
extended association - this is, in my opinion, what you saw.

Cheers, Robert Lang

(14)

Title: Types of galaxies with a very high Supernova rate

Dear SN crackers,

>I am just scanning the list (at supernovae.net) of galaxies with multiple
>supernovae observed.
>
>I am not surprised to find M100, M101, N6946 and M83 on the list - these
>are either very massive spirals with a significant disk star formation
>rate (M100, M101) or slightly-less-massive spirals with a very high disk
>star forming rate (N6946, M83).
>
>Nor am I surprised to find a lot of rather more distant "starburst"
>galaxies on the list - as these are exotic objects with an
>EXTREMELY high component of massive stars. (e.g. N2276, N3690, N664) -
>for that matter, N6754 is not at all a normal galaxy either.
>
>In the case of N1961 and N521 - these are amongst the half dozen or so
>most-luminous galaxies in the bright galaxy population - massive galaxies
>have more star formation and more supernovae, on average.
>
>What DOES surprise me is NOT finding NGC2997 and NGC1566 on the list of
>galaxies with several observed SNe, as these galaxies are comparable to
>the first four that I mentioned. I can only think that the supernovae in
>these galaxies were unobserved in the 20th C - they should have had
>several in 100 years. Same goes for NGC 1232 .
>
>Cheers, and very Happy hypernova hunting,
>from Robert Lang, extremely mad and bad galaxy person.
>
>P.S. Some SNe can be quite hard to "dig out", due to internal extinction
>within a galaxy.
>SN2002cv in N3190 had >= 7 mags of extinction, and was mainly detectable
>in the near-infrared.
>The reason I have been urging Greg to adopt slightly longer exposures in
>his search is that 1-4 magnitudes of disk extinction is a common figure
>for detected SNe, even in galaxies relatively near to face-on.
>PPS Maximize your search productivity, you "SN crackers"....preferentially
>observe galaxies with a high rate of formation of massive (> 8 solar mass)
>stars , as these are much more likely to host core-collapse supernovae.

(15)

Title: NGC 1316 - intermediate age globulars detected

Goudfrooij et al, in astro-ph/0010275, found good evidence that the
strange 8.5 magnitude elliptical/lenticular galaxy NGC 1316 on the fringes
of the Fornax Cluster has some globulars which are only 3 billion years
old, consistent with the idea that these clusters, and also N1316 itself,
were formed in a major merger between two large galaxies. N1316 may be an
example of a rather settled merger remnant, though its list of observed
morphological peculiarities is long. (accretion of smaller galaxies is a
more commonly given reason for its peculiarities)

Robert Lang

(16)

Title: M31 vs. NGC 253

Dear fellow astronomers,

The below reply comes as a result of a suggestion I made to Dr Sidney van
den Bergh that M31 is an anemic galaxy.

This galaxy class , called the "anemic" type, was originally defined by Dr
van den Bergh as -
(1) Can be a spiral of any Hubble Type
(2) Smooth arms with little star formation
(3) Redder than it should be due to a lack of current star formation
(4) Usually deficient in interstellar gas
(5) More common in the inner regions of dense galaxy clusters

As anyone who has used the telescope to make a visual comparison of M31 and
NGC253 knows, M31 is visually comparatively rather bland, with subtle dust
lanes and a smooth appearance. In contrast - NGC253 shows heavy dust lanes
and obvious spiral arms, and is rich in obvious detail even when viewed
with a 10 inch under good dark sky conditions.

And here's why.......

M31 has the following properties:

(1) It has a larger bulge than the Milky Way, but it has enormously less of
the dense molecular gas than does our own galaxy - the molecular gas is the
only gas component dense and cold enough to form new stars. M31 has a
somewhat smaller total amount of interstellar gas than does our own Galaxy
and its gas is at a much lower density - this despite M31 being a somewhat
larger system than the Milky Way
(estimates range from 0.3 to 0.7 magnitudes difference between the V band
luminosities of the two systems).

(2) Without enough available dense gas for star formation, M31 is a
quiescent galaxy which is unable to form stars at a decent rate. Its
total rate of star formation is less than one third that of our own galaxy!
(1/3 solar mass per annum in M31)

(3) Lacking large numbers of newly-formed supergiant stars, the disk (=
"spiral arm region") of M31 is therefore smooth and red, with little
current star formation. This is one reason why the spiral arms are very
hard to discern. M31 has VERY FEW young clusters with massive luminous
stars - so its spiral arms are not prominently outlined by luminous stars
and large HII regions - tending to make the arm-interarm contrast very low.
The optical colour of the disk of M31 is similar to that of an S0
galaxy.....indicating that the disk is strongly dominated by an old stellar
population!

(4) The lack of dense star-forming gas in M31 means that the dust lanes are
not heavy, as interstellar dust is evenly admixed with the interstellar
gas.( The dust lanes in galaxies are actually strings of dense molecular
gas clouds with a small amount of dust admixed)

In the August Astronomical Journal, Elmegreen and colleagues show that IF a
galaxy has either:
- used up much of its gas in previous generations of star formation
AND/OR
- been stripped of much gas (by processes such as multiple supernovae,
cannibalism of the gas by another galaxy in an encounter, galaxy
collisions, etc.)

THEN its arms are likely to be smooth and of low contrast, due to its
current lack of gas and its lack of newly formed luminous stars. Thus,
anemic spirals are simply smooth-armed gas-poor spirals.
They may in fact be on their way to becoming S0 type (lenticular) galaxies.

Therefore, I suggested to van den Bergh that M31 might be an anemic spiral
galaxy. He is thinking about it!

Cheers
Robert Lang, mad amateur astronomer

For extra credit - Some S0 galaxies (red coloured disk galaxies without
current star formation or spiral arms) contain substantial interstellar gas
which however is not dense enough to form stars. Q. What would it take to
turn M31 into an S0 galaxy?

>From: Sidney van den Bergh <sidney.vandenbergh@nrc.ca>
>Subject: M31 etc
>Dear Mr Lang,
> Thanks again for kind long letter on M31. It is nice for me to see
>that you read my books with so much interest and care. I shall add
>the list of corrections that you sent me to the (even longer) one
>that one of my (also retired) German colleagues sent me some time
>ago.
> I was VERY interested in the point that you made about the
>peculiar morphology of M31. To my eye the peculiarity looks different
>from that of "anemic" galaxies, however, I do not know how to
>account for it. Perhaps it has something to do with the peculiar
>evolutionary history of the Andromeda nebula that was first suggested
>by Ken Freeman at Mt Stromlo and was subsequently discussed in more
>detail by Brown and independently by me (astro-ph/0305042).
> Thanks again for your kind letter.
>Best regards
> Sidney van den Bergh

(17)

Title: NGC 7599 in the Grus Triplet

Dear Imagemaker,

NGC 7599 is definitely the least morphologically disturbed of the three
galaxies in the triplet.
The other two galaxies (N7582 and N7590) manifest very strong nuclear
activity - probably seyfert plus star formation burst, in both cases.

N7599 does, however, manifest a bright and small nuclear/circumnuclear
region - which is likely to host star formation. NGC 7599 has at least four
rather open arms, judging from your photograph, but does not appear to have
a significant bulge. ( a bulge being defined as a reddish extended
spheroidal structure of mainly old or intermediate age stars. )

Its Hubble galaxy classification of Sc, in the Third Reference Catalogue of
Bright Galaxies, appears to be about right. (however, a morphological type
of Sd cannot be excluded on the basis of your image of it)

My processed version of your image of NGC 7590 shows a lot more
peculiarities than are evident in your image of NGC 7599. N7590 has an
unbelievably bright nuclear region, which shows the pink of nebular
emission. Moreover, its disk contains incredibly luminous star-forming
knots in a semi-chaotic pattern, outside of which there are two reasonably
well defined spiral arms.

The Grus Triplet, incidentally, is a true physical group, and it is
embedded in inter-galactic neutral hydrogen gas, which may have been pulled
out of the galaxies by gravity.

Yours, madgalaxyman

(18)

Title: NGC 5291 and The Seashell Galaxy

Fellow galaxy fiends,

This field is in the IC 4329 cluster, also known as Abell 3574.

There is a high velocity encounter going on here between the distorted
lenticular (= type S0 ) galaxy called NGC 5291 and the adjoining
(smaller) Seashell Galaxy. Note how the surrounding field is full of tidal
material, perhaps pulled from the galaxies. There is also an enormous cloud
of neutral hydrogen gas extending through intergalactic space here.

The knotty-looking material spread throughout the field has the appearance
of very numerous irregular galaxies!!! ( they may be tidal dwarf galaxies,
formed from material pulled out of other galaxies)

The Seashell may not be an easy observation, visually, as even the
morphology of NGC 5291, the large distorted lenticular(S0) galaxy.... is
challenging to discern in the telescope.

This is just one of the many galactic "nooks and crannies" in Centaurus
which are little explored!

I just love this field, as it well illustrates what astronomy is really
about - mystery and discovery.

Regards, vmadbadastronomer

(19)

Title: NGC 1313, NGC 2997

Dear fellow galaxy fanatics,

One galaxy that seems to regularly surprise me is NGC1313. (false colour
image attached)
It is probaby roughly visual absolute magnitude -19.5, which is about 2.5 -
3.5 times less luminous than the Milky Way - so it is closer to M33 in
luminosity than it is to our own galaxy's luminosity. However, it was
recently found to contain a good dozen-or-so young and blue Super Star
Clusters - ranging in luminosity from that of the brightest open cluster
in the Milky Way through to 2 magnitudes brighter than this.
(these objects are comparable to the OB star cluster that energizes the
Tarantula Nebula, but some are even more massive - in fact, the brightest
of them are definitely within the globular cluster range of masses)

As can be seen from the large size of the HII regions (colour coded red in
the image), this may be one of the few external galaxies which are good
candidates for the visual use of narrowband nebula filters such as the
OIII. (red stellar light is coded green in the image)

Thus, considering that is is not a big galaxy, it is a remarkably vigorous
former of massive stars, and the core-collapse supernova rate may well be
higher than one would expect from the relatively modest size and mass of
the system. Also, the detectability of SNe in this galaxy will be enhanced
by the apparent scarcity of dust in its disk ( which shows that the system
has an affinity with the dwarf-irregular type) , its favourable
orientation, and also the nearness of this galaxy.

Cheers
Robert Lang, madbadgalacticperson

NGC 2997 -
A study was done recently showing that NGC2997 had at least 34 young blue
clusters of absolute visual magnitude -10 to -13. The authors suspect that
the true number, allowing for the shallowness of their observations, might
be several times this. (Larsen and Richtler, 1999, A&A, 345, 59)
(thus, the clusters range up to 16 times the luminosity of the most
luminous young open clusters in our Galaxy - OCs in our own galaxy "max
out" at absolute V magnitude -10)
Thus, there is at least some evidence for a high star formation rate in
NGC2997 - several SNe ought to have occurred in the last 100 years.

(20)

Title: NGC 6744

Dear galaxy friends,

The galaxy NGC 6744 in Pavo makes an interesting comparison with M101 - the
angular diameters are similar, but NGC 6744 has a somewhat fainter
magnitude, a lower surface brightness, and a lower rate of formation of
massive stars. A crude distance estimate of 10Mpc (33 milion light years)
can be arrived at by using the redshift of NGC 6744 and the Hubble constant.

Here is a good blue-sensitive image of it , which I scanned in from an
image by Alan Sandage. The image is attached as N6744_display.jpg

Well shown in this broadband optical-wavelengths image of NGC 6744 are the
following:

(1) The Hubble type of NGC 6744, derived from the sequence of blue-band
images that define the "classical" classification system, is SBbc(r) or
possibly S(B)bc(r), where the B in brackets connotes a relatively weak
bar. However, from perusal of the 2MASS image of it, the near-infrared
morphological type of this galaxy is SBbc(r) or more probably SBbc(rs) -
the bar is reasonably strong when seen in the absence of much of the dust
extinction, and the bar-encircling ring (or tight arms) is better defined.

There is quite a jump in the typical levels of the star-formation-rate when
we move from Hubble type Sb to Hubble type Sbc galaxies, and therefore
N6744 clearly falls into the Sbc morphological box - some Sb galaxies have
only a small amount of current star formation - a good example of one of
these quiescent Sb galaxies is M31, which has 1/3 or less of the
star-formation-rate of our own Galaxy and which has a smooth red disk with
relatively little gas and dust. (see "The Galaxies of The Local Group", by
S. van den Bergh, 2000, Cambridge University Press. )

(2) The intense, but not large, nuclear bulge - the surface brightness at
its centre is very high, as is typical for barred spirals - this could mean
that the currently high stellar-mass and stellar-light concentration here
has gradually built up over hundreds of millions of years, as gas is
funnelled in by the bar and there are successive waves of star formation in
the bulge.

(The gradual build-up and growth of the bulge of NGC 1365 through its
current and previous generations of violent star formation was recently
successfully modelled - the somewhat obsolete "old bulge" paradigm only
really works for Hubble Types S0, Sa, and Sb, but not for Types Sbc and
later. )

(3) The rather-subtle and broadly oval bar, which extends on both sides of
the bulge. (or perhaps having the appearance of two stubs which are
sometimes called protruding "ansae")
Given the detail in this image of NGC 6744, it would not surprise me if
the bar could be regarded as having some distinct morphological subcomponents.

Some commentators prefer to call an oval bar a "lens".

The difference between an "oval bar" and a normal "long bar" is that the
oval bar has axes which are more similar in length to each other.

(4) The ring, or pseudo-ring of star-forming regions that encircles the bar.
The ring in NGC 6744 may also be envisaged as being made up of tightly
wound spiral arms, but this is virtually always the case in the prominent
ring structures which are so common in barred spirals!
(this is the signature of an (r) -subtype barred spiral, i.e. a barred
spiral with a "theta'" shape - but, it is not an extremely clearly
defined ring in this case )

The enhanced star formation and surface-brightness in these bar-encircling
"resonance rings" is caused by gas piling up just outside the bar, as was
discovered by Buta, Block, Combes, Athanassoula, and others. The enhanced
surface brightness of such a bar-encircling ring is well-seen in NGC 6744,
especially when it is viewed with the eye!

(5) The multiple very-patchy arm segments found outside the ring, which are
superposed on a low surface brightness disk. This image does not show the
full enormous extent of the system of arms, which extends to some 10' from
the centre of this galaxy. (The spiral arms can be visually detected with
a 10 inch telescope - but they need a good sky. )

Because of the enormous angular size of NGC 6744, the detailed morphology
can be discerned visually - however, it is somewhat harder overall to do
this than in the case of M101 .

Regards, from vmadbadgalaxyman, galaxyco@tpg.com.au
Post Script !!!
I have just smoothed and unsharp masked the very noisy and poor-resolution
NIR image which I got from 2MASS - voila!! All is revealed...see next
email.......

(21)

Title: NGC 6744 - Continued

Dear fellow galaxy fanatics,

Here is the near-infrared view of the inner regions of NGC 6744, taken from
the 2MASS survey .....
(remember; luminous right stars and HII regions are mostly excluded in the
NIR, and dust extinction is reduced by a factor of ten when we image a
galaxy at 2.2 microns wavelength using a special NIR CCD chip)

Now the underlying mass component of this galaxy is revealed................

The idea of an "oval bar" in NGC 6744 is not necessarily excluded by this
image.

But what we see here is pretty much a normal bar, and a surrounding
pseudo-ring structure composed of two very-tightly-wound arms which seem to
almost overlap, and which seem to have their origins at the bar ends.

Excuse the poor resolution, but the main features are still obvious!

This is another good illustration of the fact that traditional blue or
visual images confuse us by emphasizing population I material - the
semi-chaotic "surface gloss" or "eye candy" of HII regions, dust clouds and
superluminous stars can confuse us as to the true structure of a galaxy!

(here is yet another reason for amateur imaging by restricting the
wavelengths observed to the very near-infared, at 750-1000nm (7500 to 10000
Angstroms) - if your CCD has sufficient sensitivity in this region of the
spectrum)

The I.N.G. (Isaac Newton Group) is developing CCDs with very high red to
near infrared (750 - 1000nm) sensitivity (80 - 97 percent Q.E. at
800-900nm) (I will send you some details shortly.) These detectors should
have many advantages over previous CCDs, as NIR shows everything you can
see at visual wavelengths - only minus some of the dust!

Cheers, Robert Lang, galaxyco@tpg.com.au

(22)

Title: Supernova search - Longer exposures needed

Hey! galaxy people.......

I have an easy paper from the ING on how near infrared CCDs are being used
to find supernovae in the heavily obscured (2-4 mags, or even more)
starburst nuclei of galaxies, many of which have theoretical SN rates of 10
per century or more. If anyone is interested in it, let me know. It is
important in SN search not to have too short CCD exposures, as many SNe can
be missed in the central regions of galaxies if exposures are too short -
specific galaxies with very high central Star formation rates would be good
candidates for long (yes, Long!) CCD exposures, in order to "dig out" the
SNe. As they note in the paper, by imaging at 2 microns only those galaxies
with an extremely high central rate of massive star formation, they expect
to discover a supernova virtually every night!

At the least, from the amateur perspective, imaging with a more red
sensitive "standard" CCD will yield somewhat superior SN detection rates,
when doing SN search of many galaxies.

Cheers, Robert Lang, galaxyco@tpg.com.au

________________________________________________________________________

(23)

Title: IC 2554 - A Strange southern galaxy

I2554pair_IIIaJ_ESO3.6m.jpg is a picture of the strange southern galaxy
IC2554., taken with the original 3.6m telescope of ESO.

This attached version of the image, stretched and unsharp-masked, shows IC
2554 as being probably one galaxy - it has sometimes been regarded as two
galaxies, perhaps in collision. A deeper image would help to conclusively
resolve this issue.

This version I have made also shows that some of the so called
"peculiarity" of IC 2554 might simply be a matter of wishful thinking. (it
also has regular gas dynamics!). It may not, in reality, be greatly more
peculiar than other barred disk galaxies such as the LMC, NGC1097, NGC 4027
and NGC 1313 - there is a significant degree of morphological irregularity
which seems to be inherent in the concept of a barred spiral.

Another attached image, I2554_IIIaJ_LocalEqualization.jpg, using a "Local
Equalization" filter, also sheds some light on the layout of the main dust
lanes in IC 2554.

Careful before you go calling a galaxy "peculiar" !! - the galaxy NGC 1313
was regarded as being highly peculiar for a long time, but it is now
regarded as being merely a normally rotating Sd ( or Sdm) with a globally
elevated rate of star formation.

Here is some information about IC2554, copied from Barbel Koribalski's
website at ATNF:

"
Abstract. ATCA HI and radio continuum observations of the peculiar southern
galaxy IC2554 and its surroundings reveal typical signatures of an
interacting galaxy group. We detected a large HI cloud between IC2554 and
the elliptical galaxy NGC3136B. The gas dynamics in IC2554 itself, which is
sometimes described as a colliding pair, are surprisingly regular, whereas
NGC3136B was not detected. The HI cloud, which emerges from IC2554 as a
large arc-shaped plume, has a size of approx. 30 kpc, larger than that of
IC2554. The total HI mass of the IC2554 system is approx. 2 x 109 Msun, a
third of which resides in the HI cloud. It is possible that tidal
interaction between IC2554 and NGC3136B caused this spectacular HI cloud,
but the possibility of IC2554 being a merger remnant is also discussed. We
also detected HI gas in the nearby galaxies ESO092-G009 and RKK1959 as well
as an associated HI cloud, ATCA J1006-6710. Together they have an HI mass
of approx. 4.6 x 108 Msun. Another new HI source, ATCA J1007-6659, with an
HI mass of only approx. 2.2 x 107 Msun was detected roughly between IC2554
and ESO092-G009 and corresponds to a face-on low surface brightness dwarf
galaxy. Star formation is evident only in the galaxy IC2554 with a rate of
approx. 4 Msun/yr.

IC 2554 and surrounds were observed withh the ATCA by Scott Gordon et al.
as part of his PhD Thesis (2002, University of Queensland). The ATCA field
consists of :
- the peculiar galaxy IC2554 (vHI = 1378+-9 km/s; various measurements for
vopt = 1240 to 1850 km/s),
- a large tidal HI plume emerging from IC2554 and extending like a big arc
towards the East,
- and the elliptical galaxy NGC3136B (vopt = 1780+-31 km/s), at a distance
of 8.2' from IC2554, not detected in HI.
- At a distance of 16.5' from IC2554 we detected the galaxy ESO092-G009 at
v_HI (ATCA) = 1450 km/s (vopt = ?), RKK 1959 at vHI (ATCA) = 1610 km/s
(vopt = ?) and an HI cloud, named ATCA J1006-6710, at vHI (ATCA) = 1720 km/s.
- For details see the paper by Koribalski, Gordon, & Jones (2003, MNRAS
339, 1203 )

" (end of quotation)

Regards,
Robert Lang
galaxyco@tpg.com.au

"The Hubble Ultra Deep Field is scientific LSD"
- R.L.
_________________________________________________________________________________

(24)

Title: Nebulae - from the Coalsack through to Circinus

Galaxy friends,

This H-alpha image of the Coalsack to Circinus region of the Galactic Plane
shows that there IS quite a lot of large-scale, but generally very
weak, nebular emission between the Coalsack and Circinus. Bear in mind
that this is a VERY deep image. (CircinusToCoalsack__Ha.jpg)

In order of increasing surface brightness :

(1) There is a very-large-scale, very-diffuse and extremely faint
hydrogen-alpha emission throughout the entire Milky Way here - some is
associated with the spiral arm which we see edge-on along here, and some is
only a few hundred light years away associated with the Coalsack and other
gas clouds in the area.
(dark nebs are in reality clouds of molecular hydrogen, with about 1% of
dust evenly admixed with the gas)

(2) The two obvious large-scale nebular emission complexes on this image
......are in reality very faint, perhaps too faint to detect visually. They
may be similar in surface brightness to the diffuse faint emission that
stretches between the Eta Carinae nebula and the NGC3576/NGC3603 complex.

(3) However there ARE a few intriguing, small, brighter spots of nebulosity
- e.g. a string of them between the Coalsack and the middle of the
Pointers. Turn down the brightness of the image a lot, using some graphics
software, and they become even more evident.
Many of these are RCW objects, and they might be suitable for visual work
or closeup imaging with the telescope . The westernmost of these small
nebs is just inside the Eastern side of the Coalsack, and is near a very
interesting "Bright-dark rim structure" made by a tongue of starry material
extending into the Coalsack.

As Georgelin remarks in her H-alpha study of the Sthn Milky Way - this
section of the Sagittarius to Carina spiral arm, which we see crossing in
front of us, is quite inactive at present when it comes to star formation
- thereby accounting for a relative lack of large and bright nebulae
between Norma and Crux. This spiral arm's nebulous component in fact
manifests mainly as a vast layer of diffuse Hydrogen-alpha emission.

Cheers
From Robert Lang

_________________________________________________________________________________

(25)

Title: Hubble Galaxy Classification - still relevant?

Dear galaxy people,

William Phillips recently asked me - what is the current status of the
Hubble galaxy classification scheme?

Here is my current viewpoint on classifying galaxies according to their
morphology:

In my view, there are now too many different types of galaxies known to
which the Hubble classification scheme does NOT apply. Further, the
classification scheme is based on a sequence of blue wavelength images -
which emphasize dust, massive stars, and therefore the current state of a
galaxy's interstellar medium - in terms of the actual mass fraction that
these galaxy subcomponents represent, they are basically "eye candy".

In contrast, Near-infrared imaging captures the light of a population which
better represents the MASS distribution of a galaxy, i.e. red giant stars.

The galaxies in the Hubble scheme are nearly all:
- of high surface brightness
- field spirals, or spirals from sparse clusters - not at all typical of
cluster galaxies
- representative of a sample of galaxies at the current state of evolution
of the universe.

The second point can be well illustrated by viewing a really good modern
image of the rather dense Antlia cluster of galaxies, which is relatively
nearby in space, at about twice (or perhaps 2 and a half times) the
distance of the Virgo Cluster -
It is disconcerting to find that many of the galaxies in this more dense
environment are hard to fit into the Hubble scheme!

Historically, because the well-studied Virgo cluster galaxies are closer in
properties to those galaxies in the low-density "field" distribution of
galaxies... than are the galaxies in the more dense, but less studied,
Fornax cluster - we ended up with a classification scheme emphasizing
galaxies found in low density regions.
(even now, the distinctive mix of galaxy morphologies in the denser Fornax
cluster has had only two or three studies )

The list of galaxy-types and galaxy morphologies which are NOT in the
Hubble scheme is now very long - even if you use some "upgraded" Hubble
schemes which incorporate several successive refinements and several
successive extensions.
( mostly the work of Alan Sandage, Gerard de Vaucouleurs, and Ron Buta)

Thus, it is questionable whether the very long list of diverse
morphological types and subtypes ought to be further extended.

A symptom of this excessively large number of morphological categories in
the Hubble Classification is that in order to fit REAL galaxies - very
diverse, and very complex - into specific morphological type boxes (by
assigning each a Hubble type), the latest iterations of the Hubble Galaxy
classification resort to nearly unpronouncable Hubble Types such as -
R'SAB(rs)b II (smooth arm)(very early)
Further, there is no single and widely-accepted classification scheme for
S0 type (lenticular type) galaxies, and the elliptical galaxies are now
split into three or four subtypes.

A partial list of galaxies and galactic phenomena which are hard to fit
into the Hubble scheme is:

(1) Low luminosity dwarfs which are mainly composed of HI and dark matter,
with the stars comprising only a very small fraction of their total mass. A
good southern example is NGC 2915. There is also the possibility of the
existence of dark matter concentrations without any gas or stars attached
to them (i.e. "Dark Galaxies").

(2) The low surface brightness galaxy population - with disk surface
brightnesses which can be up to 4 magnitudes down on the typical spiral
included in the Hubble classification scheme.

(3) Spirals within dense clusters of galaxies, which tend to have very
smooth arms.

(4) A newly discovered subclass of the Sa galaxies, with the expected
smooth arms devoid of much star formation, but which nevertheless
unexpectedly have very-significant current star formation within them -
however the star formation in these Sa's is hidden deep inside the bright
bulge and inner disk component of the galaxy. (generally, this is mainly
visible in H-alpha images).
This significant minority of Sa galaxies having HIGH star formation rates
- in this "hidden" region - were not foreseen in the normal classification
scheme, as this did not utilize Hydrogen alpha imaging.

(5) Spiral Galaxies at distances of 5 billion light years and more, which
show evidence of their disk component still being in the process of
assembly. (the disks are raggedy, entropic, and often asymmetric. )

(6) Possibly more than one type of S0 (=lenticular) galaxy, e.g. some
having very small faint disks and thus resembling elliptical
galaxies, some having large-scale disks and thus resembling spiral
galaxies, some S0 galaxies having a significant ISM, and other S0s having
virtually no Interstellar Medium.

(7) Ellipticals with disky (pointy) elliptical isophotes vs. elliptical
galaxies with boxy elliptical isophotes.
Ellipticals turn out to be a diverse population, in terms of their stellar
orbits and their 3D shapes - elliptical galaxies exist which, for instance
- include a small disk component, are rapidly rotating, are slowly
rotating, rotate around the long axis, are oblate spheroidal, are prolate
spheroidal, are triaxial ellipsoids.

(8) Systems resulting either from the merger of two large galaxies, or from
the successive accretion of one or more small galaxies by a larger galaxy.
(e.g. NGC3256, NGC3610, NGC5266, NGC1316, NGC520, NGC5128)

(8) Numerous exceptional systems, e.g. galaxies where the long axis of
their bulge is at a large angle to the angle of their disk ( e.g. NGC4698),
dust lane ellipticals (e.g. NGC5266), galaxies having rings of matter which
revolve about the primary galaxy on a polar orbit (e.g. N4650A, NGC 660),
galaxies with blue bulges (e.g. NGC 5102), etc.

(9) A large and complex family of bar structures and ring structures within
galaxies - at all spatial scales ranging from that of the immediate
circumnuclear region to the that of the outermost regions of galaxies.
For example, NGC 1566 and M94 have overall oval-shaped disks (which may be
desribed as a "fat bar"), NGC1291 has a small bar nested inside its
large-scale bar, the outer arms of NGC1300 bend around to form a complete
ring, etc.

(10) There is also some evidence that spiral galaxies as a population may
not be intrinsically circular - at approximately the level of having a mean
intrinsic axial ratio of 0.94

When you add these new families of galaxies and galaxy structures to the
already numerous Hubble Types which are already in existence, you can see
the problem with creating ever larger numbers of discrete galaxy classes
(types) and galaxy subtypes.

In my view, therefore, an analysis that classifies galaxies by splitting
each galaxy into a relatively manageable number of discrete morphological
and kinematically-defined and age-defined subcomponents may be of more
value than trying to come up with ever-expanding lists of new "types" of
galaxies. For instance (this is not a precise analysis, but indicates the
sort of thing I mean) -

Milky Way classification = Central Black hole + central starforming region
+ bulge + bar + star forming ring in inner disk? + disk (= young disk + old
disk) + thick disk + optical halo + dark matter halo + infalling gas from
the halo or from elsewhere in Local Group

M87 classification = central 10E9 mass black hole + circumnuclear disk
+ central mass and light concentration + bright stellar spheroid + faint
optical halo + metal-rich globulars + metal poor globulars + dark matter
halo + dark matter distribution of surrounding galaxy cluster

In other cases, a morphologically bland and SEEMINGLY simple galaxy can be
split into subcomponents by considering its component stellar velocities
(kinematics). For instance, the Virgo cluster E3 galaxy, NGC 4365, is
composed of two different stellar subsystems which are NOT evident on
inspecting an image of it - the central region of this galaxy rotates
about the short axis of the galaxy, while its outer parts rotate around the
long axis of the galaxy..... In this case, it is the stellar velocities
that give away the two discrete components!

Cheers
Robert Lang (alias, madbadamateur)
_____________________________________________________________________________

(26)

Title:
Is astronomy a dangerous game?

Dear galaxy friends,

Astronomers are most generally believed to have a sedate lifestyle and to
have an excellent occupational health and safety record -

But consider the sad fate of the following professional astronomers, who
were all top-notch contributors to our field:

Marc Aaronson was killed by an observatory dome - as he opened a door near
to the edge of the dome, the dome was supposed to stop rotating - it didn't
do this, and the door crushed him, with fatal consequences for him.
Aaronson, then in his 30s, was already one of the top extragalactic
astronomers of the 1980s.

Jean-Luc Nieto, a really excellent French astronomer, met with a fatal
accident while climbing the North face of Pic du Midi, in the Pyrenees.

Jeff Willick, an expert on peculiar velocities of galaxies, was killed by
impact with an out-of-control sports car, as he sat working with his laptop
while in a cafe.

I also have heard of an astronomer who was killed by a cerebral oedema, due
to the high altitude of Mauna Kea.

Astronomy is obviously a much more dangerous game than I thought it was!!!!

Cheers
Robert Lang
______________________________________________________________________

(27)

Title:
The Pavo/Telescopium/Indus galaxy cluster complex

>The NGC6876/NGC6872 group of galaxies in Pavo is sometimes called the
>"Pavo Group", but this is not at all helpful - as there are several other
>galaxy clusters in Pavo of similar richness and distance.
>
>The question of whether this is a poor cluster or a rich group is an
>academic one, but the luminosity function appears to be normal, as there
>are numerous small galaxies in the group.(some of which are not even in
>the PGC/LEDA database - typical spotty data for far southern objects)

>The Pavo Supergalactic Structure, which contains the N6872 galaxy group,
>is at about 4000km/s redshift - it is a physically real complex of at
>least 6 clusters plus intervening galaxies in Telescopium, Indus and Pavo,
>and which then stretches through Ara to Triangulum Australe, where it
>joins the somewhat obscured (but extremely rich) cluster Abell 3627. (see
>"Large-Scale Structures In The Universe" by Anthony Fairall)
>
>In imaging these regions, it is not uncommon to find fairly bright
>galaxies which have not even made it into the PGC/LEDA catalogue... they
>are very poorly surveyed.

Often, even entire southern Abell (ACO) galaxy clusters fall off the
"mental map" of Northern professional astronomers - for instance.... AJ
125, 478 (2003) purports to be a study of the brightest cluster ellipticals
in a nearly complete sample of nearby Abell clusters - but the authors,
when having to leave out a few clusters from their study due to time
constraints, choose to point HST at the northern or equatorial objects in
preference to the Pavo/Indus/Telescopium objects - and this, despite the
extreme lack of extant data for the far southern objects!

>Other similarly excellent observing targets in the Pavo/Telescopium
>complex of galaxy clusters and galaxy clouds include:
>
>(1) Probably the least known of the constituent clusters of the Pavo
>supercluster, the cluster ACO 4881 in Pavo, centred on the galaxy
>IC4765. It has not even been studied in the literature till the last 3 or
>4 years, despite being an excellent cluster for the astrophotographer and
>for the visual observer!! This is a typical situation for the rich galaxy
>fields of Pavo and Triangulum Australe in general - due of course to their
>southern Declination.
>
>(2) The NGC 6868 cluster (N6868, 6861, 6870, etc)(= Abell 4927)(=G80)(LGG
>430), centred on the giant elliptical/S0 transition object NGC6868....
>which has had a handful of studies so far. Historically, Alan Sandage
>named this group the Telescopium Group - which is arguably the name that
>should be used for this object - however, there are actually three
>prominent groups/clusters in Telescopium, all of them of similar
>prominence, so the name is actually highly confusing.
>
>(3) The N6707 / IC 4797 / IC4796 / ESO183-30 cluster in Telescopium.
>Again, detailed imaging of this scattered cluster is hard to come by. It
>contains a non-NGC/IC galaxy which is of visual magnitude 11.6 !!
>
>(4) The N6769/NGC6770/IC4845/NGC6782 cluster in Pavo. Some wag even gave
>this cluster the designation "Not E"....galaxy cluster nomenclature is an
>absolute mess!
>These galaxies are similar in their distances and in their apparent sizes
>to those in the NGC6872/NGC6876 group. N6769/6770/6771 is a very good
>triple system which includes tidal material.
>
>(5) Abell 4916. An interesting field surrounding NGC6848 = PGC64023 =
>ESO185-52, and ESO185-54 = PGC64041, which are two 13th B-magnitude
>bulge-dominated galaxies of high luminosity.
>
>(6) The "Indus I" cluster (= ACO 5000). Its brightest galaxy, at around B
>= 13, is NGC7014. However, there are several apparent and also
>rather-distended groups or subgroups scattered over a wide area of sky,
>e.g. near the B=13 magnitude galaxy ESO235-55, which is one degree away
>from NGC7014.
>The similar redshift galaxies in this area are remarkably scattered, and
>it is hard to explain how such very extended structures have formed.
>
>Despite the recent explosion in the imaging capabilities of amateur
>astronomers, it is still remarkable how little-known are many of these
>wonderful southern fields. I might also remark that for many members of
>the bright southern galaxy population, we are STILL stuck with grainy
>low-resolution Schmidt images when we want to study their morphology!
>
>Robert Lang

_____________________________________________________________________________

Title:
What is an S0 (lenticular) galaxy?

Essentially , type S0 galaxies are a heterogenous bunch, with many having
little neutral hydrogen gas, and some having almost as much as a spiral.
(albeit at densities too low to be able to form more than a smattering of
new stars)

Most lenticular galaxies (to use the word preferred by de Vaucouleurs'
disciples) have a very dominant old stellar population, but some of them do
also contain many stars of intermediate age. (i.e. 3-6 billion years old).

However, within the uncertainties, "old" could mean 8 billion years old or
it could mean 12 billion years old.

The stellar ages in S0s are not at all well known, as they are virtually
never studied, when compared to the trendy Ellipticals and spirals. I
don't know why - their origin and morphology is a fascinating unsolved
problem.

A significant minority of S0s do host immediately noticeable current star
formation, but mainly within the bulge. (this is also the case for some
apparently quiescent Sa galaxies with smooth arms)

In addition, some S0s are "spiral-galaxy-like", e.g. with a significant
disk component and a bar structure. While others are "elliptical galaxy
like" with a bulge component and the merest hint of a disk.
(some ellipticals in fact contain extremely faint disks INSIDE the bulge
component)

It is not clear how S0s formed - some may have formed by stripping of gas
from spirals in clusters. Others may have formed in the early
universe. There is plenty of evidence for both views of their origins.
They can also be formed (in simulations) from the merger of two other
galaxies.

When S0s are imaged in detail. (which is only now starting to happen), the
impression one gets on viewing the image is - "the shock of the unfamiliar".

M31 will turn into something like an S0 if it uses up much of its supply of
interstellar hydrogen.
It is gas poor , already. However, there is new evidence recently that the
ISM of galaxies can to some extent be recharged by gas clouds falling in,
literally from intergalactic space. Some of the High Velocity Clouds of
neutral hydrogen have now been shown to be falling into our own Galaxy,
either from
the extreme margins of its halo, or from even further afield! (one of them
is probably dominated by dark matter and not HI gas - curiouser and
curioser, said the white rabbit)
__________________________________________________________________________

(29)

Title: The Milky Way is NOT 100,000 light years across!

A correspondent wrote to me:

>The figure of 100,000 LY across arose from research in the 1950's - it has
>probably been picked
>up and repeated mindlessly, as these things often are. Much of the
>"generally accepted" knowledge
>is probably in this category. However there are no signposts at the edge
>of the disk - as the
>attached photo indicates ,the halo may be very extensive. You are probably
>right in
>terms of (the Milky Way containing) "not much" beyond 80,000 LY, but there
>may be outlying stars and gas and dust that, however tenuous (no pun
>intended), leaves an argument for a larger number.

I replied:

The region of strong spiral structure in our Galaxy extends only to radius
= 12 kpc (40,000 light years).

In fact, the Perseus spiral arm, outside of our own location in the Galaxy,
is already MUCH weaker than the next arm inwards from the Sun. (the
Sagittarius-Carina spiral arm). This is quite evident from comparing the
visual surface brightness of the Milky Way in these two arms!

It doesn't take a genius to see that we are further out in the Galaxy than
we thought we were -
(solar radius = 8.5kpc, versus a galaxy radius of 12 kpc)

All you have to do to appreciate where we really are in the Milky Way is to
compare the faint Perseus Milky Way with the much brighter Milky Way
between Scutum and Carina!

Also, most studies show that there is much lower dust extinction outside of
the solar circle - less dust means less gas, and therefore less star
formation.....Here again, all you have to do is to compare the smooth
northern milky way with the much more irregular-looking southern milky way,
by eye, in order to appreciate this fact.

cheers, Robert Lang, mad galaxy man
__________________________________________________________________________

(30)

Title: Fine structure within a bright southern elliptical galaxy?

The southern elliptical galaxy NGC 6876 may contain substantial fine
structure.

I had first noticed in Jim Riffle's image of the NGC6876/NGC6872 group in
Pavo, that this giant elliptical system possibly has some fine structure
within it.

The inner region of this galaxy is noticeably more elliptical than its
outer region, something better shown on your image. In fact, as one goes
out from its centre, the isophotes (lines of equal brightness) seem to
change from being round to being notably elliptical, and then to change
back to being round again.

This could be the sign of an inner disk structure, or other inner
substructure, within this elliptical - these sorts of things have been
confirmed in other ellipticals, by means of detailed isophotal analysis.
( a Local Equalization Filter, applied within Corel Photopaint, seems to
resolve the "inner oval" into something like a spheroidal bulge plus a
flattened disk..... WITHIN the "inner oval region")

There also seem to be traces of some other smaller structures, in Jim's
image, but it is hard to say how real they are - more to the point, I don't
want to believe in them, unless I have a better image of NGC6876!!

The role of subtle fine structures in ellipticals is currently a hot topic
in astronomy - most big ellipticals seem to have experienced, at the very
least, an event in which they have cannibalized another smaller galaxy,
leading to the presence of certain subtle inner stuctures. (and a few have
definitely formed through the merger of two large galaxies).

NGC1316 is a famous southern example of the process of galaxy accretion
and/or merger, and is an E/S0 system showing obvious fine-structure. An
equally weird example of a galaxy merger is the southern dust-lane
elliptical NGC5266 - but in this case it would seem that the system is
almost certainly the result of the merger of two large galaxies. (in
Centaurus A, in contrast, a giant elliptical has swallowed a low-mass spiral)

Believe it or not, ellipticals do contain extremely-subtle flattened disks
in some cases.....but these disks are very small and very faint. (imagine
an S0 with a very small disk....then make the disk even smaller, and you'll
see what I mean). The most common manifestation of this is when we find an
E5 or E6 galaxy which is slightly pointy at the two ends of the major axis.

Incidentally, N6876 also has an enormous , faint halo of light which
encompasses it and its companion galaxy. (not unusual for supergiant
cluster ellipticals).

Regards, Robert Lang
__________________________________________________________________________

(31)

Title: Observing Dwarf Galaxies in the Sculptor Group

Fellow galaxy hunters...

The Sculptor group of galaxies is so nearby that it is easily possible to
observe the brightest of its ~16 known dwarf irregular galaxies.

The five brightest irregulars or very-late-type spirals in the Group
are (in no particular order) -

ESO 245 - 05 = PGC 6430

NGC 625

NGC 59

NGC 24

NGC 45

Note : The membership of NGC 24 is contentious, as it may be in the
background of the group. However, it is relatively close to NGC 45, both
in angular terms, and in radial velocity.

Just for fun, I include a B + R + H composite image of NGC625 - the B
photometric filter image is coded blue, the R photometric filter image is
coded green, and the H-band (NIR) image is coded red.
The strongly reddish cast comes from a lot of near-infrared light from the
old stellar population - so this is not a true colour image. This system
looks more like an S0 in NIR light only, while it looks like a dwarf
irregular in blue light. So it may, possibly, be an "amorphous" , i.e. a
starburst dwarf S0 galaxy - but I'd have to check on this.

Note that dwarf irregulars and very-late-type spirals (Sd-Sdm-Sm) are
generally the most common galaxies on the fringes of galaxy groups, and in
the lowest density regions of space. How it is that very-isolated dwarf
irregular galaxies are able to form at all, given the very low matter
density of their typical environment in space, is an interesting question.

Several dwarf irregular galaxies in the Sculptor Group were recently
studied in (2003) AJ, 125, 593

The Sculptor Group of galaxies has had a lot of study recently, mainly by
Cote, Freeman, Binggeli, Jerjen, Karachentsev, and their associates. There
is now good evidence that it is more likely to be a freely expanding
galaxy-cloud or association, than to be a true gravitationally bound group.

However, true Bound Groups do exist WITHIN the overall Sculptor Galaxy
Cloud , e.g. a probable group made up of NGC55 plus NGC300 plus several
very faint companions.

Cheers
Madbadgalaxyman

Incidentally, the LMC-type system NGC 55 , according to Karachentsev et al,
has a reasonably reliable distance estimate of 5.4 million light years.
(A&A, 389, 812). However, as anyone who has observed NGC 55 knows, its
high degree of resolution is similar to that of the prominent dwarf
irregular galaxy NGC 3109 which is at 4.3 million light years
distance. Optimistically, the error on these distances is +/- 10 percent.
But +/- 15 percent may be more realistic.

Evidently, NGC 55 is much closer than NGC253..... which is at the far end
of the Sculptor Cloud of galaxies - however, as at a year ago I was unable
track down a reliable distance for NGC 253.

Fortunately, as was found by the above paper, and also by the group of
astronomers associated with Ken Freeman, the distance-velocity relation is
very well defined and also very linear for the Sculptor group of galaxies -
so the velocity distance of NGC 253 is still quite plausible - a distance
of 12.5 million light years, assuming a local Hubble Constant value of 73
as applied to its recession velocity from the centroid of the Local Group
of Galaxies. Note that the value of 2.5 Megaparsecs (8 million light
years) distance for NGC253, which is often quoted, was an indicative figure
only.

If anyone can improve on the enormous uncertainty in the N253 distance
which existed one or two years ago, let me know. (the possible distances
ranged from 8 million light years to 14 million light years! )
________________________________________________________________________________

(32)

Title: "The FIR brightest" Galaxy Catalogue - the weirdest of the weird.....

Aficionados of peculiar and interacting galaxies -

Take a look at the new IRAS Revised Bright Galaxy Sample (and
mini-galaxy-catalogue)

This is in - the Astronomical Journal, 126, 1607 (2003)

This sample and catalogue comprises the 600-or-so most Far-Infrared-bright
galaxies in the sky.

"This is really really beautiful and useful data" (as I once said to the
very nice brazilian lady astronomer who sent me data on Abell 4881 )

- there is a table which gives the estimated FIR luminosities of the
galaxies and also other info - FIR luminosity correlates with star forming
rate, so this information will help you to isolate peculiar and
starbursting and interacting galaxies. The good thing is that you don't
have to work out the far-infrared luminosity of a galaxy, as it is already
done for you!
And if you like extremes, just observe the galaxies with the highest FIR
luminosity - some of the most powerfully luminous objects in the local
universe. (well within the quasar range of luminosities, and NOT half way
across the known universe)

(did you Know that UGC 8058 = Markarian 231 is the most FIR bright object
in the sky? - look up table 1, for many similar gems. The AGN at the centre
of this galaxy has a quasarlike luminosity)

- there is also a very useful image file, which contains small versions of
DSS images of all of the galaxies in this far infrared catalogue. (it is
really interesting to view 600, often very peculiar, galaxies in a single
image file)

The images of the "600-galaxy FIR brightest sample" are a rogues gallery
which includes some of the weirdest galaxies in the sky.

The sample includes great numbers of VERY peculiar systems such as N6240
and N5394/N5395 and N4038/9, and also systems with high to very high (but
not extremely high ) star-forming-rates which are however interesting due
to their proximity e.g. M101, NGC4945, N1448, N2442, N4151, N5728, N1365

Cheers, Robert Lang
_______________________________________________________________________________

(33)

Title: NGC 59 - probably not a dwarf irregular.

Dear galaxy friends

I stated in a previous communication that NGC59 is a dwarf irregular.
( I got this classification from a scientific paper - but it isn't correct)

It is probably not a dwarf irregular galaxy!!

It is of a dwarf S0 morphology, however, it is also the host of significant
star formation.
Thus, a type of "amorphous" or "IO" may not be out of the question.

Oddly, the presence of significant current star formation is common in
dwarf S0 galaxies - at least, the 20 or so examples that I know all do have
evidence for some star formation either currently or in the last 200
million years.(M82 is arguably a dwarf S0 undergoing a violent central
burst of star formation.)

(and S0 galaxies were supposed to be "past it" as far as star formation goes)

An image of NGC59 can be found in AJ, 116, 2873 (1998)

An individually measured distance ( SBF method) of 4.39 Mpc exists for NGC 59

Regards, Robert Lang

_____________________________________________________________________________

(34)

Title: The Centaurus Galaxy Cluster, Down to 14th magnitude

Dear galaxy colleagues

I attach a Word 2000 format document, "Centaurus Cluster Galaxies.doc",
which lists the 24 brightest galaxies in the very galaxy-dense core of the
Centaurus Cluster of galaxies - this is that region which is near to the
giant ellipticals NGC4696 and NGC4709.

NGC 4696 and NGC 4709 are the chief galaxies of two subgroups of the
Centaurus cluster, separated in recession velocity by 1500 km/s! However,
these two subgroups do NOT appear separate, when viewed from our
line-of-sight....

It is thought that , despite the large difference in velocity, the two
subgroups are at the same distance, and that one group is falling into the
other at high speed.

The attached list extracts all galaxies to Blue magnitude 14.6 from a
Catalogue of the Centaurus Cluster published by H.Jerjen and A. Dressler in
1997. (A&AS, 124, 1)
They observed and classified all the galaxies on an 100 inch du Pont
Telescope plate.

Their survey was limited to an 1.5 by 1.5 degree region which is near to
the two big ellipticals, so it does not include nearby galaxy-rich areas
such as the Centaurus Chain of galaxies.

The galaxies in this field are many, but usually faint - there are only a
few of them to a visual magnitude of 12.5, and they only start to become
really numerous at 13-13.5 magnitude and beyond. (the attached list
contains the 24 brightest systems in the field)

As remarked in the attachment, imaging of this field is challenging - the
cluster galaxies only start to become really numerous at a typical galaxy
diameter of 1.2 - 1.4 arcminutes and less.

Cheers, Robert Lang
_____________________________________________________________________________

(35)

Title: The Centaurus Cluster - the available catalogues

dear fellow galaxy fanatics,

Remarkably, I was unable to find a modern catalogue of the entire Centaurus
Cluster complex at the CDS.

So, it would appear that we are stuck with the comprehensive catalogue of
Jerjen and Dressler for the Central region, and that of Dickens Currie and
Lucey (mid 1980s) for the entire cluster.
(the refs are given in the Word 2000 document attached with the previous email)
(Dickens et al ......is on some strange magnitude system which I'm still
trying to work out! )

This is the sort of cluster survey project that could be done with CCD
imaging by an amateur astronomer - Jerjen and Dressler were going to do the
entire cluster in the late 1990s, but were stoppped by the closing down of
the photographic camera at the du Pont100 inch telescope!

However, there IS a nice catalogue of the Hydra ( = Hydra I) cluster by
Hamabe et al, which is available to view or download at CDS.... ApJS, 85, 249

Just make sure to download the readme file as well - there are a lot of
fields in Hamabe et al's catalogue.

Regards
From madbadastroman
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(36)

Title: The Centaurus and Fornax Clusters - How far away?

Dear galaxy people

Just for "fun and amusement", I include a closeup of part of the supergiant
elliptical galaxy NGC 1399 , which is near the centre of the Fornax
Cluster. The peppering of small dots is part of the very rich (very
roughly 7000 members in all) system of globular clusters associated with
this galaxy. Note also the dense distribution of dwarf galaxies. This is a
section of an image that was taken with the MOSAIC camera at the 4m
Telescope at CTIO, by Boris Dirsch and colleagues. Very large numbers of
even fainter dwarf galaxies are resolved in Dirsch's other image of
NGC1399, taken with the VLT !!

Boris Dirsch and colleagues (AJ, 125, 1908) found 6450 +/- 700 globulars as
an estimate of how many of these clusters NGC1399 has, and they also found
that NGC 1399 has a globular cluster luminosity function consistent with a
distance of 62 million light years (19 Megaparsecs) for this galaxy.
However, take this distance estimate for the Fornax Cluster with a grain of
salt - despite persistent efforts to refine the distance of the Fornax
Cluster, repeated distance measurements made during the last 15 years still
arrive at a wide scatter of distance estimates for the Fornax Cluster of
galaxies - usually within the range 52 - 68 million light years.

(essentially meaning that Fornax could be at 1 to 1.2 times the distance of
the Virgo Cluster - a very unsatisfactory situation! )

More on the distance of the Centaurus Cluster -

As mentioned last time, a low-end distance estimate for the Centaurus
Cluster of galaxies is 110 million light years. (roughly twice the distance
of the Virgo Cluster).

However, several teams have found distances of up to 140 million light
years, using other techniques, e.g. Mieske and Hilker found a distance of
41.3 Megaparsecs ( = 135 million light years) using the Surface Brightness
Fluctuations distance-measurement technique. They also find a probably
significant difference between the line-of-sight distances of NGC4696 and
NGC4709, which are the chief galaxies of the two subgroups in the cluster
that are known to be separated by 1500km/s in recession velocity - this
would not be surprising, as the higher velocity subgroup is believed to be
falling into the lower velocity subgroup, thus accounting for the velocity
difference. (see their preprint at astro-ph/0309680 - found at
www.arxiv.org )

The presence of significant substructure, and of a non-compact spatial
extent in the line-of-sight, is very likely for the Centaurus Cluster - the
presence of several subclusters, and of a significant line-of-sight
extension in space, is already quite well studied in the Virgo Cluster of
Galaxies.

(in contrast.... the Fornax Cluster is a VERY compact cluster of galaxies)

If you compare;
- the sizes of the galaxies,
- the distribution of apparent magnitudes of the galaxies,
in the two clusters (Virgo and Centaurus), you will be forced to admit that
2.5 times the Virgo distance is a very plausible distance for the Centaurus
Cluster.

One should bear in mind that a lot less work has been done on The Centaurus
Cluster than on the Coma Cluster of Galaxies - it took decades of work to
come up with a compromise distance of about 300 million light years for the
Coma Cluster!

Cheers, Robert Lang

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(37)

Title: How To Increase your rate of Supernova discoveries

Greetings, my fellow "SN crackers",

Over the last year or so, I have been working on two significant problems
with your currently adopted methods of CCD-based SN searching. These
problems with your current search methods will, at least to some degree,
result in a significantly sub-optimal detection rate of core-collapse
supernovae by your search programs:

Problem (1) -
The inclusion in your list of search targets of too many specific galaxies
having star formation rates which are only low-to-moderate, and by
extension....the EXCLUSION from the target list(s) of too many galaxies
which are known to have high to very-high star formation rates. For
instance - dwarf and also relatively low-luminosity galaxies are not worth
searching , and some of the Sc galaxies that you do search will have LOWER
star formation rates than some Sa & Sb galaxies which are excluded from the
search programs.(as was shown conclusively by recent H-alpha and Far
Infrared galaxy surveys)
The formation rate of massive stars in a specific galaxy can be predicted
quite well from several quantities that can be found in galaxy catalogues
for that galaxy - so that once I calculate the star formation rate in each
catalogued galaxy , I can confidently PREDICT which specific galaxies found
in the catalogues are most likely to have a high time rate of core-collapse
( e.g. Type II) supernovae. Thus, the actual SN detection rate in your
search programs can be considerably increased by means of a more careful
choice of which specific galaxies to include in your regular imaging program.

Problem (2) -
The presence of substantial dust extinction within the spirals which you
are imaging.... leads to a situation where the shallow magnitude limits
that you adopt in your imaging of these spirals are probably too bright
for you to be able to detect a significant fraction of the Type II
supernovae that actually do occur in these galaxies.

Problem (2) is not quite as bad as I thought it would be, but you still
have to allow for about half to three magnitudes of dimming of the SN, due
to dust in the SN's host galaxy - I am still investigating exactly how
much extinction can be expected , however. ( A CCD chip with very good red
to near-infrared sensitivity can help to reduce the amount of observed
extinction in front of an SN, as extinction gets less with increased
wavelength )

I currently have problem (2) under investigation - it is not easy to
predict how much extinction will, on average, be in front of type II SNe,
as caused by the dust in front of an SN from the galaxy which hosts the
SN. I am reading a dozen or so recent papers on the effects of dust within
galaxies - and there are still numerous contradictory claims as to the
typical amounts of dust and extinction found in spiral galaxies !!

My motivation is partly to assist you, and partly just to satisfy my
natural curiosity about the interstellar medium in galaxies.....

I should eventually be able to calculate a rationally-derived magnitude
limit for your imaging, which will ensure that the majority of the
core-collapse SNe that do occur in your searched galaxies ARE detected in
your images - I suspect that your observations are currently too shallow to
pick up considerable numbers of the type II supernovae occurring in the
galaxies that you are imaging.

Cheers, Robert Lang, madbadastroman

When told that I have been studying dust in galaxies , some wag recently
suggested to me - "Why don't you study dust in the carpet instead?"
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(38)

Title: The galaxy NGC 1365

Dear fellow galaxy trekkers,

The recent CCD surface photometry of NGC 1365 by L.M. Macri et al, ApJS,
128, 461 (2000) finds that NGC 1365 has, in total, an apparent B
(approximately Blue) magnitude 10.01, and that it has an apparent V
(approximately visual) magnitude of 9.51.

These B and V magnitudes yield an apparent B-V colour index of 0.50 for
this system, which is indeed a MUCH more blue colour than is usually
expected for very luminous spirals like NGC 1365. (the more massive the
galaxy, the redder it tends to be, on average - at least for a fixed Hubble
type)

In contrast, you will find a total B magnitude of 10.32 (from aperture
photometry) given for NGC 1365 in the Third Reference Catalogue of Bright
Galaxies. But much more troubling is the fact that the RC3 gives a MUCH
REDDER overall galaxy colour of B-V = 0.69 for this system...

The very large total mass of this galaxy, together with the newly-found
more blue colour (which would indicate that it has a high massive star
fraction), could easily explain the high rate of observed supernovae in NGC
1365. However, the newly found very-blue B-V colour of B-V = 0.5 seems TOO
blue to be plausible for a large spiral ! Of course, if this colour IS
true, NGC 1365 would be a remarkably vigorously star forming galaxy.

IN CONTRAST, a completely separate magnitude measurement, by Tully and
Pierce,
[ ApJ, 533, 744 (2000), Table 1 ] finds a total B magnitude of 10.24 for
NGC 1365 - illustrating very well the amount of relative difference which
can still sometimes be found between two very careful sets of magnitude
measurements!! A much more serious discrepancy is the fact that Tully and
Pierce find that the apparent B-R colour of NGC 1365 is 1.29, which
corresponds to a colour of about B-V = 0.78. Thus, they find a global
colour for this galaxy which is rather red - therefore, overall, Tully and
Pierce agree with the Third Reference Catalogue colours, and also with the
B-R colour that is found in the ESO-Uppsala Surface Photometry
Catalogue(the ESOLV).

Macri et al. also find that N1365 is a little bit bigger than is usually
assumed - the standard major axis diameter on the RC3 system was found by
them to be 11.5 arc minutes.

Incidentally, NGC1365 has an extremely bright Far-infrared apparent
magnitude of 8.24. This shows that it has a total star-formation-rate
which is considerably higher even than the star formation rates of the
first-ranked (and high SN-rate!) Virgo Cluster spirals such as M100 and
M61!! However, much of the star formation in NGC 1365 is obscured by dust
- because there is a powerful starburst in the nuclear or central region
of this galaxy.

Unfortunately, really ratty catalogue data is a fact of life - and
especially for many prominent southern spiral galaxies.

However, the large discrepancy that exists between the derived colours of a
well-known galaxy, as measured by different workers all of whom are
experienced and careful observational astronomers, is much more troubling -
as the numerical value of a galaxy's optical colour is one of the
fundamental parameters which can be used to the derive the star formation
rate of that galaxy.

Cheers, from madbadgalaxyman, galaxyco@tpg.com.au

P.S. The inclination (usually measured from ZERO degrees when a galaxy is
at a face-on orientation, through to a 90 degrees value when a galaxy is
edge-on) of NGC 1365 is also notoriously poorly known. The isophotes
(ellipses of constant apparent brightness) of this galaxy have yielded
inclination measurements in the range of 44 to 63 degrees from face-on, in
various references. The kinematically measured inclination of NGC 1365
(measured from the rotation of NGC1365) is 40 degrees from face on - and
this is likely to be near to the correct value.
The "spanner in the works" could well be the fact that the inner disk of
NGC 1365 is actually intrinsically oval. Of course, various warps and
asymmetries can also be complicating facts in the measurement of inclinations.

Be aware that measured spiral galaxy inclination values can easily be out
by anywhere up to 10 degrees....
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