NGC 3603 - Open cluster or Young Globular....or What?

[madbadgalaxyman]

The first section of this post is copied from an email I wrote about some of the unusual types of massive (at least 10,000 solar masses) and compact and centrally-concentrated Star Clusters that are not the familiar Globular Star Clusters with their exclusively old (9-13 billion year old) stellar populations. There are several different types of these Extremely Rich and Very Centrally Concentrated star clusters that look a lot like globulars, but which are not exactly globular star clusters.

Then I include an MPIA press release, which tries to answer the following question:
"will the young star cluster NGC 3603, which is already looking a lot like a globular, eventually turn into a more familiar 'old' globular star cluster?"

Then I attach a review paper about Nuclear Star Clusters, which are another variety of rich, massive and centrally-concentrated Star Cluster that greatly resembles a globular.
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From madbadgalaxyman's email:

At about 10,000 solar masses or more (as being the value of the total cluster mass), young and intermediate-aged star clusters in our own Galaxy and other galaxies, adopt a steep and centrally-concentrated Radial Profile of surface density..... which is similar to the radial profile that is observed in the canonical "old" Globular Star Clusters.
[[ “surface density” (in professional astronomical jargon) refers to the apparent two-dimensional density of stars or of surface brightness, rather than the actual three-dimensional density of the stars in a cluster ]]

NGC 3603 is an example of this type of cluster. Other clusters of this type include:
- the Very Compact cluster R136 at the core of the Tarantula Nebula
- Westerlund 1 (which is significantly obscured, and in our own Galaxy)
- the Arches and Quintuplet clusters that are near to the center of our own Galaxy.

Another object which is arguably a low-mass Globular Star Cluster that is not old......is Messier 11 (this object is normally regarded as a high-mass open star cluster.)

These objects are often called "Super Star Clusters" or "Massive Compact Young Clusters” in the literature, but essentially they can be thought of as looking a lot like young globular star clusters.(normal globulars are some 9-13 billion years old, which is nearly as old as the oldest stars in the universe).

It would seem that these possible"young globular clusters" can be formed just about anywhere that the pressure & density of the cold molecular ISM reaches a very high value, for instance:
- where the ISM of a galaxy gets a mighty shock that is caused by a galaxy collision or a galaxy merger (e.g. in NGC 4038/9 and in NGC 3256)
- in the largest molecular gas complexes, which can most commonly be found in the "starburst" regions at the centres of barred spiral galaxies and in the spiral arms of non-dwarf Sc galaxies.

(( A good example of a galaxy with some intermediate aged (<7 billion years old) globular clusters is NGC 1316, which has undergone one or more mergers with other galaxies. ))
(( Some tentative evidence has been produced for the existence of massive & compact & centrally-concentrated young "globular-like" Star Clusters of up to 100,000 solar masses within the galaxy M83, but I don’t know if there is any recent work that has 100% confirmed this idea.))

A related type of star cluster, which again looks very similar to an ordinary globular star cluster, is the "nuclear star cluster" which is often found at the very center of many galaxies (including our own!). Nuclear Star Clusters can be a lot more massive and luminous than even the most massive “old” globular star clusters, and nuclear star clusters can contain stars of various ages. NSCs are easiest to see in type Sd galaxies (e.g. NGC 300 and NGC 7793) , due to the very light screen of dust that exists within this type of galaxy.

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MPIA Press release about NGC 3603
(this press release is not 100% true to the actual science results, but it is better than nothing!!)

WILL NGC 3603 BECOME A GLOBULAR CLUSTER ?

Stars in Motion: High precision follow-up study of star movement shows surprising unrest in massive star cluster

Using the NASA/ESA Hubble Space Telescope, astronomers from the Max Planck Institute for Astronomy in Heidelberg and the University of Cologne have completed a long-term study of one of the most massive young star clusters in the Milky Way, comparing two observations that were made ten years apart. The comparison, which relies on extremely precise measurements, reveals the motions of several hundreds of stars, which prove to be at odds with current models of how such clusters evolve, stellar motion not having “settled down” as expected. The results have been published in the Letters section of the Astrophysical Journal.

Ordinary star clusters ("open star clusters") disperse over time, as the different stars go their own separate ways. Very massive and compact clusters are thought to be different. In the long term, this can lead to the development of massive aggregations of stars known as "globular clusters", whose tightly-packed stars remain gravitationally bound to each other for billions of years.

With a mass of more than 10,000 suns packed into a volume with a diameter of a mere 3 light-years, the massive young star cluster NGC3603 is one of the most compact stellar clusters in the Milky Way. (For comparison: in our own immediate stellar neighborhood, the same volume contains no more than a single star, namely the Sun.) Could this be a globular cluster in the making?

To find out, a team of astronomers led by Wolfgang Brandner (Max Planck Institute for Astronomy, Heidelberg, MPIA) tracked the movement of the cluster's many stars. Such a study can reveal whether the stars were in the process of drifting apart, or about to settle down. It also serves to distinguish members of the star cluster from unrelated stars that, as viewed from Earth, just happen to fall along the same light of sight.

By using two observations, made ten years apart with the same camera aboard the Hubble Space Telescope, and by performing an intricate analysis to account for all possible disturbances, Brandner and his colleagues were able to reach the required accuracy.

All in all, the team observed more than 800 stars. About 50 of these were identified as foreground stars, which are unrelated to the cluster. From the remaining sample of more than 700, the astronomers were able to obtain sufficiently precise speed measurements for 234 cluster stars of different masses and surface temperatures. Boyke Rochau (MPIA), the paper's lead author, who performed the data analysis as part of his PhD work, explains: "Once our analysis was completed, we reached a precision of 27 millionths of an arc second per year. Imagine you are in Bremen, observing an object that is located in Vienna. Now the object moves sideways by the breadth of a human hair. That's a change in apparent position of about 27 millionths of an arc second."

The results for the motion of these cluster stars were surprising: According to widely accepted models, which reproduce what is actually observed in older globular clusters, the average stellar speed in a cluster like the one in NGC3603 should depend on mass: Stars with lower mass should move faster, and those with higher mass should move more slowly. The stars for which precision measurements were possible represent a range of masses between 2 and 9 times that of the Sun. Yet all of them move at about the same average speed of 4.5km/s (corresponding to a change in apparent position of a mere 140 micro-arc seconds per year). Average speed does not appear to vary with mass at all.

Apparently – and surprisingly – this very massive star cluster has not yet settled down. Instead, the stars' velocities still reflect conditions from the time the cluster was formed, approximately one million years ago. Team member Andrea Stolte from the University of Cologne explains: "For the first time, we have been able to measure precise stellar motions in such a compact young star cluster. This is key information for astronomers trying to understand how such clusters are formed, and how they evolve."

Vexingly, the question of whether or not the massive young cluster in NGC3603 will become a globular cluster remains open. Given the new results, it all depends on the speeds of the low-mass stars, which were too faint to allow for precise speed measurements with the Hubble Space Telescope.

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Here is a review paper about Nuclear Star Clusters, from IAU Symposium S266 in the year 2009:


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You will note, from reading this paper, that there are many different types of small spheroidal stellar objects, ranging up to the size of dwarf spheroidal galaxies!!

See also this conference:

http://moca.monash.edu/conferences/compact _____________________________
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[Robh]

Hi Robert,

Very interesting.

It always strikes me how much M11 looks like a globular cluster in binoculars.
Not that it should make it one.

As much as we define the properties of an object, there exists objects that exhibit properties from different sets. For example, is the massive Omega Centauri the remains of a dwarf galaxy?

It can be difficult to differentiate clusters into open and globular clusters in nearby galaxies such as the LMC and SMC and M33.

Good read.

Thanks, Rob

[madbadgalaxyman]

Rob,
I am glad you enjoyed my post.

The "open vs globular" distinction may turn out to be somewhat artificial, with the type of structure existing in each cluster dependent mainly on its total mass.

M33 has been proved to contain some globular star clusters which are much younger than the old globulars that are found in our own Galaxy.

In my view, a certain number of the most massive of the star clusters existing in the LMC (e.g. NGC 1846) are probably best thought of as being recently (relatively recently) formed Globular Star Clusters. A recent paper gave an age of about 2 billion years for NGC 1846......not exactly very young (!!), but 5 or 6 times younger than the canonical "old" globular cluster population that is found in most spiral and elliptical galaxies.
Here is a very interesting study of the star cluster population of the LMC:
http://arxiv.org/pdf/1207.5576v2.pdf
The young and intermediate-age Star Cluster population of the LMC is well populated at very large masses e.g. 10,000 to 100,000 solar mass clusters!!
(note: If anyone needs help understanding "the jargon" in this paper, I can help to decipher it!)

It is thought that at least the most massive of the newly-formed and compact Star Clusters that are known to exist in various environments (e.g. within the 'starburst' regions at the centres of spiral galaxies, in galaxy collisions, and in the largest gaseous complexes within the spiral arms of galaxies), with these clusters weighing in at some 100,000 solar masses per cluster, could well evolve into ordinary "old" globular star clusters, given enough time.
However, if NGC 3603, which is currently structured like a globular cluster, gradually loses an appreciable fraction of its 10,000 solar masses of material due to some of its stars exiting from the cluster, it will probably turn into an ordinary open star cluster....... but its future evolution is not well constrained, as yet.

The formation and time-evolution of the most massive newly-formed star clusters has been an area of strong controversy in the professional literature; I am not sure of the latest results in this area.

The star-formation rate of the Milky Way Galaxy (conventionally expressed in units of "solar masses per annum") is very ordinary compared to the star-formation rate of moderately active Starburst Galaxies like NGC 253 and NGC 1365;
and (in general) the relative numbers of stars that form at many & various specific values of Star Mass are the same, independent of environment (( technically, this is known as the invariance of the stellar Initial Mass Function (an invariant IMF) ))
Therefore, relatively moderate starburst galaxies, which have some 5 to 15 times the star-formation rate of our own Milky Way Galaxy, will form a whole lot more supermassive OB stars than our own Galaxy forms.......
therefore, while the population of Star Clusters in our own Galaxy is "baseline truth", we cannot extrapolate the properties of these local Clusters to other galaxies.

cheers,
Robert

[Weltevreden SA]

It will probably be awhile before NGC 3606 YC can be recommended as a candidate globular in the usual sense of a compact gravitationally bound stellar object. Presently 3603’s C-M diagram is a near linear track straight up the MS; few of its stars have moved off the main sequence. Its 130+ very massive O and somewhat less massive B stars have not reached the turn-off point. There are three Wolf-Reyets, though, the same number as the 5-times large R136 cluster in the Tarantula Nebula, but 3603’s are a full magnitude brighter. UV radiation from the O and B giants have cleaned out most residual star-forming gas; we can readily see this in visual-band astro images of the cluster. Equally important to the professional astronomers with 8m class equipment, the hot O and B winds smooth reddening across the core, or rather de-lumpify it, which makes distancing more accurate. Most of 3603’s <Msol stars appear to have been formed primordially out of a collapsing original gas cloud of 10,000+ Msol; the O and B stars then formed dynamically from cloud shocking during a rapid mass collapse in the core; the time scale between these two starform periods is uncertain but likely abrupt. The two nearby dust pillars we see in published images on the S and W are succumbing to the withering UV bombardment they’re getting from the cluster core. In half-million to a million years will come the shock and MHD turbulence of multiple core collapse SNs, probably a large number of them in close succession. The agonies of M82 show what happens to a galaxy-scale object after a large number of closely spaced SN spasms; now imagine what’s in store for this rather modest cluster. 3603’s overly bright future is to be expected given the near-instantaneous core formation documented by Brandner’s 2012 paper.* When SN shocking does occur, the combined supersonic shock wave and interstellar gas heating will considerably alter the gravitational gradient of 3603’s remaining sub-8 Msol stars; the likely scenario will be something akin to a core collapse globular on a modest 5000-star scale.

*Robert, were you citing Brandner's 2012 (http://arxiv.org/abs/1204.5481v1) paper or the 2010 (http://arxiv.org/abs/1006.0005) paper?

Presently 3603 lies between the Carina Arm and the denser Scutum-Centaurus Arm of the galaxy. As it transits to the next arm it will encounter collapsing new clusters and high-velocity molecular clouds whose younger cousins we see so readily as dark nebulae in wide-field astrophotos of 3603’s surroundings today. Presently there is a moderate overweighting of O versus B stars in the SW quadrant at sub half-parsec distances; the Bs are more evenly spread in the E and N. The O stars will SN first. While we don’t know how the SNe in 3603’s <2myr future will affect the cluster’s galactic orbit, a sector-specific SN light-up could nudge the cluster into a lissajous orbit above and below the disc like the Sun experiences over its 2.3 gyr cycle. This would add orbital crossing shocks to 3603’s already fulminous future.

Hence, as we observe it today, we are seeing a doomed cluster about to lose mass on a significant scale. (And I think MY diet is awful!) First will come core-collapse SNs and expulsion of regional star-forming gas; then core collapse of the low-mass stars remaining; and finally, in the really long term, PN gas expulsion as a population of WDs emerge. All these factors appear to preclude 3603 becoming a GC. It might end up a dense open cluster of the NGC 6719 type, with a negligible surface evaporation of old low-mass stars and WDs shocked by density wave and disc crossings.

I am not sure about the MPIA article’s interpretation, ‘a mass of more than 10,000 suns packed into a volume with a diameter of a mere 3 light-years’. Brandner’s paper isn’t about size or astrometry considerations, it’s about the cluster’s primordial or dynamical segregation and timescale (http://iopscience.iop.org/1538-3881/132/1/253/fulltext/). For population, stellar class, and distance accuracy, I would stick with the Melena, Massey et al 2008 (/iopscience.iop.org/1538-3881/135/3/878?fromSearchPage=true) paper, which uses spectroscopic parallax, main-sequence fitting, and kinematic distancing based on the MW rotation, to establish 3603’s DM at 18.7-19.1. Also see Stolte and Brandner et al 2006 (http://iopscience.iop.org/1538-3881/132/1/253), which directly compares 3603 with the R136 cluster in 30 Doradus in the LMC, a cluster pair that is five times as massive and >50 times more spread out. There are so many papers about 3603 it is entertaining to check out the chronology of all the et.als who have chanced on this visually unimpressive cluster. That’s how I discovered it, too: by accident.

The discussion about 3603’s classification serves to put Robert’s point in a larger picture: things are getting so complicated that old, easy paradigms are on shakier ground by the day. For example, the term “globular cluster” no longer means a disc or halo aggregation of metals-thin Pop II stars with a large C-M variance. NGC 1818 (http://arxiv.org/abs/0903.4787) in the LMC is classified as a GC even though it is less than 40 myr old and has a complex near-solar metals content. The matter of 3603’s brand-label classification category is modest compared with its origin, evolution till now, near-term future, and above all, what we can learn about it as we glimpse it through our modest backyard observer eyes, AP gear, and spectroscopes. Consider what 3603 and its associated giant H2 complex and the Tarantula Nebula in the LMC must look like to some home-brew astronomer with a 300 mm scope living on an inconspicuous (and hopefully not cloud-bound) planet somewhere in M33: Pretty much what NGC 604 and NGC 588 in M33 look like to us. There’s a little Herschel in all of us who look up into the night and never come back.

[madbadgalaxyman]

Here is a 2004 review paper about Young and Massive & Compact star clusters in galaxies, by Soren S. Larsen:
[ This is catalogued at //arxiv.org as the preprint astro-ph/ 0403244 ]

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And here is a study of two galaxies (NGC 1569 and NGC 1313) which are known to contain newly-formed star clusters having masses of 100,000 solar masses or more;

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The following are a few points which I have extracted from Section 3 of A.Adamo et al. (2011, MNRAS, Vol.417, p.1904) , regarding the circumstances in which particularly massive star clusters can form:

- Mergers of two galaxies produce more numerous and more massive clusters than quiescent spiral galaxies.

- Another effect that leads to more high-mass clusters forming is: a galaxy that forms very large numbers of star clusters will also form greater numbers of star clusters that are of very high mass.

- As one might expect, the total number of young and massive star clusters that are found within a galaxy is likely to be positively correlated with the total star-formation rate of that galaxy; a larger total number of formed Star Clusters will, all else being equal, lead to a larger number of massive clusters existing.
For instance, some studies have found that the brightest star cluster within a galaxy is likely to be outstandingly luminous if the host galaxy has a particularly high star-formation rate; a positive correlation has been found between the star-formation rate of a galaxy and the absolute brightness of its brightest star cluster.

- Here is Figure 6 from the paper by Adamo et al. , demonstrating the strong positive correlation between the Star-Formation Rate of the host galaxy and the visual luminosity of its brightest star cluster:

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- Shear forces within rotationally-supported Disk Galaxies (such as spirals) favour the formation of low-mass star clusters, as this encourages fragmentation of Giant Molecular Clouds.

- The lack of rotation in dwarf irregular galaxies, and the high gas pressures in merging galaxies, tend to favour the formation of very-massive star clusters. While not all Dwarf Irregular galaxies have a lot of star clusters, it is remarkably common to find a dwarf irregular galaxy that contains one or two very-massive star clusters. (an easily observable example of a dwarf galaxy with a single super-luminous star cluster is NGC 1705 ).
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Bad Galaxy Man's comments of the day:
(1)
You will note that some of the star clusters plotted in Figure 6 are easily as luminous as an entire dwarf galaxy! The most luminous of these star clusters have a luminosity of absolute V magnitude –17, which is over 100 times the luminosity of the most luminous globular star clusters that are found in M31 and our own Galaxy.

(2) Regarding cluster formation in dwarf galaxies;
Certainly, there are many dwarf irregular galaxies which have one or two supergiant nebular/star-forming complexes that can potentially contain very-massive new star clusters; a good example of this is the small irregular galaxy known as IC 4662 which has two complexes that are approximately on the same scale as the Tarantula Nebula.
On the other hand, contrary to the appearance of the High Surface Brightness examples of dwarf irregular galaxies that we are all very familiar with, the majority of dwarf irregular galaxies are actually of extremely low surface brightness, with an extremely low star-formation rate.
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[madbadgalaxyman]

Dana,
Thank you for your detailed comments and thoughts about NGC 3603. I am glad you are investigating this object in greater detail, as I always have the tendency to very-quickly go back to looking at issues from the Galactic and extragalactic point-of-view.

I do agree that N3603 could lose a lot of its mass, with the passing of time. The 100,000 solar mass Young Clusters found in some other galaxies are therefore much better candidates for eventual evolution into a typical "old" globular cluster.
On the other hand, the mass function (distribution of mass) of the population of old globulars (which is remarkably invariant between many and different galaxies) includes many globular star clusters of surprisingly low mass.

I shall consider further, but I have very little free time at the moment!

cheers, robert

As you say, the term 'globular cluster' has become difficult to define. Because of this, in the literature, when this term is used without any further qualification or definition, it has come to mean only the population of 10-13 billion year old clusters that has nearly the same properties in virtually all elliptical and spiral galaxies; the luminosity function of the clusters, the metallicity distribution of the clusters, the cluster sizes and central-concentrations, etc., have been found to be pretty much the same, wherever we look in the universe. (although there are nearly always two distinct populations of old globulars).
So the term 'globular cluster' is still helpful, in that it is still used to refer to two unique & closely-similar populations of very old clusters, that are found in many and various galaxies.

[madbadgalaxyman]

See also, a study of star cluster and HII Region formation in the remarkable little dwarf galaxy IC 4662 ( which is an easy object to see visually in dark skies) ........
arXiv:0806:2302 at //arxiv.org

[madbadgalaxyman]

Actually, the relevant section of my post was citing a 2010 paper by B.Rochau et al., which includes Wolfgang Brandner as one of its authors. But this section was not written by me!, so its inaccuracies are due to the fact that it is a press release from MPIA ( which I believe is Brandner's institution.)

The 2012 paper you mention looks very interesting, as the formation of these very massive clusters is so poorly understood; the theorists virtually could not get one of these clusters to form in their simulations, due to the drastic effects of stellar winds and photon pressure from multiple OB stars.

"Just for fun", here is a picture of NGC 3603 taken with the Hubble Space Telescope and its ACS/WFC instrument, displayed here in a reduced-resolution version. It was taken with the F435W and F550M and F850LP filters, which correspond approximately to the B and V and SDSS z bandpasses.
(display coding is B + V + I = blue+green+red)

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[madbadgalaxyman]

Hello, all you Young Massive Compact Cluster aficionados,

Here is a paper giving individual stellar photometry for the stars in some particularly-massive (100,000 solar masses and more) star clusters in the galaxies NGC 1705, NGC 1313, Messier 83, NGC 7793, and NGC 1569.

http://arxiv.org/pdf/1106.4560v1.pdf

Studying individual stars within star clusters, in galaxies out to a distance of 16 million light years, is a very hard thing to do, even when using the Hubble Space Telescope.......0.05 arcsecond angular resolution is just barely good enough to make a start on such a project!

In observational studies of extragalactic star clusters, 0.05 arcseconds has to be regarded as extremely poor angular resolution.......

Each pixel of the detector corresponds to about 3 light years in a galaxy that is at a distance of 13 million light years, so it is hard to understand how these authors could be sure that they are detecting individual stars, without contamination from other stars......therefore I do have very strong doubts about the truth of this paper.

[Weltevreden SA]

03/31/2013:

Thanks to Robert for posting the beautiful HST image of 3603. To my taste, it’s the best of all of them all associated with this fascinating object, Maybe it has to do with the spider diffraction spikes that show up on so many of the HST’s ACS/WFC imager. With our long histories at the library poring over books of images from the likes of Mt. Wilson up to the 8m giants, and as many hours at the eyepiece peering past Newt spider spikes, maybe we’ve just become accustomed to them and think of them as sort of beauty marks. As Larsen puts it in his 2004 paper, ‘It is a human habit to characterise those things with which we are most familiar as normal.’

Well, ‘normal’ or not, I am always excited at diffraction spikes in an astro image, distracting though they may be (the details of the HST’s NGC 6741 white dwarf images are negatively affected by so many spike crossings).

Anyway, before getting on with the topic at hand—the properties of cluster formation as described in the papers Robert cites—I have a question about the many so-called ‘star lines’ in the HST 3603 and so many other astro photography images I see. (Maybe it should be thread all its own in the ‘Observational and Visual Astronomy’ forum.) Counting very quickly, I see around 20 apparent lineations of 4 or more stars of perhaps 2 mag difference with respect to each other, marching in all directions across the 3603 image. Giant globulars are even worse. I know they are my mind’s eye trying to enforce order in what I know to be chaos, and that they can’t possibly be genuinely linear bound-objects. Not in clusters whose real-time orbitals mimic the boggling chaos of an N-body simulation. I don’t see this effect at the eyepiece to nearly the same degree, though ‘threads and filaments’ are frequent phrases in my logs of globulars. (Check out NGC 5460 (http://www.univie.ac.at/webda/cgi-bin/ocl_page.cgi?dirname=ngc5460)—an OC that at the eyepiece looks like a snake charmer did it.) At one time I thought ‘star lines’ might by MHD artifacts—stars lining up with magnetic field lines. But that’s grasping at straws, because field line strengths in galactic arms are on the order of 3 to 5 microteslas, which would barely bestir an iron filing here on Earth, much less a 0.1 – 10 Msol string of stars in a massive cluster. Anyone care to weigh in on this illusion?

Of Robert's citations, the Bastian & Adamo M83 paper was the most informative and cogent. It’s going to get another read, yellow high-lighter at the ready. (And, of course, at the eyepiece if and when the current jet-stream drench above me heads over towards you folks.)

Larsen’s papers are very good. Thorough within their self-defined limits and do not either muddle with too much detail or overstretch their conclusions. He also makes the best use of the Harris Property which categorises cluster luminosity divided by host galaxy luminosity x 100. It’s a handy shorthand when describing objects so diverse as star cluster masses.

OTOH, I had a problem with the Adamo, Ostlin et al 2011 paper’s management of cluster diversity. Table 1 reveals a selectivity problem: they quote mostly their own research. That’s understandable, because they chose three unusual, atypical objects, Blue Compact Galaxies. There’s not exactly been a rush to the observatories to chase down these three; they are certainly not as sexy as NGC 3603 or dwarf irregulars. Table 1 appears in § 3, ‘Relations Between Cluster and Star Formation Rate’. The discussion that follows is more like an analysis of the vexatious diversity of clusters as a broad category. I kept thinking as I read the ifs-ands-&-buts, ‘Why don’t they just say that massive clusters in compact galaxies get that way because there’s nothing to stop them?’ No high-velocity infall, no dark matter halo, no territorial disputes with their flatmates (there aren’t any), no apogalacticons with anything, no rotational instability. A lot of gas, a large gravity well, and no competition—what more could a massive young cluster carving out territory ask for? I longed for a word about dark matter potential, but they didn’t address it—probably because no one has asked for a radio study of BCGs. Maybe I’m being superficial and obtuse about the paper, but I came away looking for the kind of picture that I find so readily in papers about dwarf galaxies and their environs. To be fair, though, the trio does state their case early: ‘. . . shear in rotationally supported galaxies (i.e., spirals) acts on the collapse of the giant molecular clouds (GMCs), causing fragmentation and favouring the formation of the less clustered OB associations and low mass clusters . . . The lack of rotation in dwarf galaxies and high external pressures in merging systems favour the collapse of massive and gravitationally bound cluster.’ I just wish they’d had an editor with an iron red pencil.

Clusters are like herding cats. I followed up on the subject using The Usual Suspects (arXiv, IOP, Scholarpedia) and came away longing for a couple of hours with my cat snoozing on my lap. And me with it.

So thanks again to Robert for all the leads.

=Dana

[madbadgalaxyman]

The unusual structure of NGC 5460 looks physically real to me, rather than to be an artefact of the human imagination. Here is an image of it, from wikisky.org:

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In the olden days, I used to visually observe a lot of OB associations, objects which are (by definition) gravitationally unbound; and OB associations often have an elongated or linear structure, similar to what is seen in NGC5460.
(( I have briefly covered OB associations in two threads within the Science Forum, entitled: "OB associations - a quick introduction" and "OB associations - another perspective" )

[[ A good case study of the way in which the human imagination can get in the way of a proper analysis of physical reality is the case of the large scale structure of the galaxy M31.....people kept looking for large-scale and high-contrast spiral structure in this galaxy; therefore their conceptual/imaginative prejudices meant that (for several decades) they didn't notice that the strongest structure within the Disk Component of M31 is actually a supergiant-sized ring! ]]
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I have (in visually observing) previously looked at the "nearby" elliptical galaxy PGC 50448 = ESO 221-026 ......
but in true "galaxyman" fashion I have always absolutely ignored the star cluster!! (as per the following anecdote about my "galaxies obsession")

True Anecdote from a star party:

Observer 1 (waving his arms in excitement, and literally screaming with joy) :: "There's a -1 magnitude comet blazing in the sky!!#$%%^!!!!!"
Madbadgalaxyman (as he peers at a very faint galaxy with his telescope) :: "Tell this to someone who cares! I won't let this unimportant announcement distract me, even for a second, from viewing this vanishingly faint galaxy!"

[madbadgalaxyman]

Dana, and other "supermassive" young cluster people,

Here is a very recent, comprehensive, and very long (80 pages!!) review paper about "Young Massive Clusters"/"Massive Compact Clusters"/"Super Star Clusters"/"Young Globular Clusters"

This paper is the following preprint: arXiv:1002.1961

This paper was also published, in its final form, in the Annual Review of Astronomy and Astrophysics:
Zwart et al., 2010, ARAA, 48, 431 (But the final version could cost you money!)

http://arxiv.org/abs/1002.1961

Have you got enough data yet, my fellow Star Cluster aficionados?
I hope this paper gives you a pleasant case of "data overload"!
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And here is even more and more data......
http://iopscience.iop.org/1538-3881/140/1/75/fulltext
which is a recent study of the very-rich system of newly-formed Very Massive star clusters belonging to the "two galaxies collision" system NGC 4038/9
These authors find that the most massive of the newly-formed young clusters are well in excess of a million solar masses per each cluster; it is hard to avoid the conclusion that some of the most massive of the new clusters which were formed in this galaxy-collision will eventually turn into "old" globular clusters. (The most luminous of these newly-formed clusters are about absolute V magnitude -15)

Here is the beginning of this paper, formatted as a Word 2000 Format (.doc) document:

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[Weltevreden SA]

I think I'm going to move to another town and start over as a Boy Scout with a pair of binoculars!

[madbadgalaxyman]

Refer to the recent Astro-imaging Forum post about Steven's fantastic Near-infrared image of the Nuclear Star Cluster at the centre of NGC 7793

[madbadgalaxyman]

Continuing on from Dana's post, in the astro-imaging forum thread about NIR imaging of NGC 7793 and its central star cluster;
(discussion moved here, for greater relevance)

Dana's post:
Robert, you might want to have a look at the 2010 Portegies Zwart paper arXiv1002.1961v1. It's specific to the MW but the IMF data on young massive clusters is extensible to many 'coreless' galaxies like 7793 with poor evidence of a supermassive black hole.

galaxyman replies:
Thanks for the reference!

I own Four very-nice & Very Up-to-date books on stellar populations, so I may abstract - for the benefit of IIS members - some of the information about the existence of multiple populations of stars (with differing ages and metallicities & helium content) found within the same globular star cluster.....the list of such clusters that are known in our own Galaxy continues to grow ever larger, well beyond the initial findings of several unique Main Sequences in the cluster Omega Centauri.
This work, on multiple populations within the same star cluster, could be relevant to trying to decipher the (probably) Very Complex evolutionary history of the central star clusters found within galaxies.

Recent work, mainly by people associated with Boker, indicates that while Nuclear Star Clusters can get much more massive than globular star clusters, they still have the same "characteristic or 'virtually constant' physical size" that characterizes many of the compact & massive Star Clusters that are known.
( As has been said in the literature, some of the most massive globulars in our galaxy may have originated as the nuclei of cannibalized Dwarf Elliptical galaxies )

Incidentally, the Sd Hubble galaxy classification (for instance of NGC 7793 and NGC 300) has little physical meaning for the central regions of galaxies, because the central radial Surface Brightnesss profiles of Sd galaxies are diverse and puzzling; this part of the Hubble Sequence is more distinguished by its "sub-giant" galaxy luminosity and by the rather chaotic & entropic spiral arms, than by any particular central structure.
[ For that matter, a distinct Hubble Class, when assigned to a specific galaxy, does say something worthwhile about its spiral arms, but says very little that is definite about the prominence of its central bulge; there is no unique or characteristic bulge-to-disk ratio for a given Hubble type!! This is why the recent De Vaucouleurs Atlas of Galaxies only gives a Hubble classification for phenomena in the extended and easy-to-see disk components of galaxies. The sizes and shapes of bulges cannot be characterized qualitatively (they cannot be estimated, just by inspection of a galaxy image), so the bulge properties of galaxies must be measured!]