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madbadgalaxyman
04-06-2014, 01:56 PM
The following overviews, resources, scientific papers, etc., I recently compiled for our resident "globular cluster fanatic" Dana, as being an "extension" to his interests in clusters and stellar evolution. But I may as well list them here, so they can be accessed by all and sundry!

These are amongst the best papers, from my last few hundred downloads.......
the impossibly large numbers of papers constantly added to arxiv (the preprints server) are probably due to the unprecedentedly large number of bright young astronomy PhDs and post-docs who are just entering the field. I have been intensively following research astronomy for some 23 years, from a total period of 40 years interested in astronomy), and I can say that there have never before been anywhere near so many young workers entering the field at one time.
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1) Attached is a really useful and really recent overview about the bulge of our close neighbour and old friend, M31.
163610
The actual published reference is: 2013, PASA, 30, 27
(Proceedings of the Astronomical Soc. of Australia)
Neither bulge nor halo of M31 are like that of the MW, and its disk is also entirely different (M31 disk is gas poor, with low intensity of star formation; )
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(2) Attached is a very concise, and easy-to-read, review paper about the bulge component of our own Galaxy.
163609
This is: Minniti & Zoccali (2008) in IAU Symposium 245 "Forrmation and Evolution of Galaxy Bulges"
Unfortunately, the information about the stellar populations within the bulge is already (!!) out of date. In particular, Ken Freeman and Joss Bland-Hawthorn have whipped up a storm of interest in accurately characterizing and understanding the nearest bulge.
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(3) This ARAA 2012 review , by Ivezic & Beers and Juric, about the disk and bulge of our own Galaxy, is relatively accessible, and it at least attempts to explain the concepts.
http://www.astro.washington.edu/users/ivezic/Publications/publishedIBJ2012.pdf
(3 Megabyte .pdf file)
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(4) Here is a very detailed overview of the stellar disk component of our own MW galaxy.
It is not quite as overspecialized as Freeman's recent intricate efforts based on radial velocity and spectral surveys of our Galaxy, therefore it is more suitable for the "lesser brethren", but I still find parts of it to be difficult reading!
http://link.springer.com/article/10.1007%2Fs00159-013-0061-8
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(5)
Certain new species (?) in the zoo of massive stellar aggregations that are not quite clusters and not quite galaxies (or perhaps a bit of both) : :
http://adsabs.harvard.edu/abs/2012A%26A...547A..65B
(http://adsabs.harvard.edu/abs/2012A%26A...547A..65B)As Dana said, when we look outside of the fairly tame (= with a low & steady Star Formation Rate) environment of the local regions of our own Galaxy, star cluster studies get very complicated....because there are a great variety of star cluster types that we are not really familiar with.
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(6)
And yet more complexity in the cluster zoo...... the Nuclei of elliptical galaxies.....
These objects perhaps DO NOT seem to follow the size and luminosity distributions, and the scale relations, of normal Compact & Massive Star Clusters like globulars:
http://adsabs.harvard.edu/abs/2012ApJS..203....5T

But see also:
http://arxiv.org/abs/1205.5338
This paper provides comprehensive data on Nuclear Star Clusters in galaxies, star clusters which for the most part seem to resemble more massive and more luminous versions of standard globular clusters, BUT with younger populations of constituent stars.
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(7)
The (relatively) young Globular Clusters found in the galaxy NGC 4636:
http://adsabs.harvard.edu/abs/2012ApJ...759..116P
This is a relatively new discovery (unlike in the merger remnant NGC 1316, where the youngish GC population has long been known). This paper also has a good summary of previous work done on the GCs of giant elliptical galaxies.
Incidentally, Kormendy and Fisher and Cornell and Bender, in their fantastic catalogue of the morphology and photometry of Virgo Cluster elliptical galaxies( 2009. ApJS, 182, 216), suggest that NGC 4636 is actually a face-on S0 galaxy (a disk+bulge system).
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(8)
The star clusters in the intensely star-forming spiral galaxy M83::
Here are four Studies of its population of massive star clusters, deriving their ages and masses (etc.) , and done with the Hubble Space Telescope. It would seem that this galaxy does have youngish (= less than a few times 100 million years old) star clusters of 100,000 solar masses each, and also a few star clusters which are even more massive than this, which is as you would expect if there were larger numbers of giant nebular complexes than in our own galaxy. In general, the disk S.F.R. in M83 is thought to be well elevated over the value that we find in our own Galaxy, but it is not a well studied parameter. There are some tentative hints in this work that, perhaps, the most massive new star clusters (one million solar masses, and up) may not form in the disks of spirals (for the most part), but only form in galaxy collisions and in dwarf irregular galaxies.

http://arxiv.org/abs/1402.3595
This team promises to publish a study of the entire M83 disk population of star clusters that are over 5000 solar masses.

http://www.aanda.org/articles/aa/abs/2011/05/aa16206-10/aa16206-10.html
This paper also studies the cluster system of NGC 1313, a remarkable nearby relatively high-surface-brightness Sd galaxy which is undergoing a global burst of star formation. One of the most interesting galaxies in the sky for visual observation!

http://adsabs.harvard.edu/abs/2011arXiv1106.2427B (http://http://adsabs.harvard.edu/abs/2011arXiv1106.2427B)

http://iopscience.iop.org/0004-637X/719/1/966
The mass distribution of the youngish clusters along one of the two main spiral arms of M83 is well studied in this ApJ paper.
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madbadgalaxyman
04-06-2014, 05:03 PM
A few broken links were fixed!

Weltevreden SA
06-06-2014, 08:45 AM
Hi all, back amongst the internetted after a week of wondrous nights with no electricity. Robert's post has some v.good papers, & kudos to him for doing all this homework for us. Paper #3 is a good summary of where MW studies are these days; it's one of those print-&-bind papers destined to be ready reference for years to come. It introduces the nifty idea that 'dimensions' can be more than 3D plus time, and discusses what some of them can usefully be. I would never have thought of metallicity as a 'dimension', but this paper shows how it is so.

Renato might want to check out paper #5 by Brüns & Kroupa. It directly addresses Renato's point about the seemingly arbitrary boundary of open -vs- globulars which oversimplify the myriad variations on stellar density rates (clusters). Skip over the technical jargon and just read the ideas glimmering under the jargon. It seems all these pricey mountaintop observatories are paying down their mortgages with data that take us way beyond the parochial suburbs of the Local Group. It's a very different world out there than it was a mere 5 years ago. Plus, as Robert points out, all that young PhD energy is a lovely sight for we who fret over the future of the science.

madbadgalaxyman
06-06-2014, 06:58 PM
Renato,

as Dana says, the question "how many ways can nature make a massive (10000 solar masses or more) and centrally-concentrated 'globular-like' star cluster?" is hard to answer.

In my opinion , nature can certainly make "brand new" globular clusters in galaxy merger events, as there is now abundant observational evidence - in several well-studied elliptical galaxies - that there do exist genuine globular star clusters that are substantially younger than the canonical "9-13 billion years old" old age of the globulars found in the MW and M31 and M81 and M104.
(NGC 1316 has plenty of these intermediate-aged globulars, and NGC 5128 has a few)

It is also confirmed that a merger between two large galaxies can result in the recent formation of young star clusters that weigh in at 1 million solar masses (or more) per star cluster. Well studied examples are NGC 4038/4039 and NGC 7252.
[[ Many of the star clusters formed in these collossal galaxy collisions will actually dissolve, eventually (Dana knows a lot more about star cluster dissolution than I do) ]]

The Nuclear Star Clusters (NSCs) which are often found at the very centres of types Sc to Sm spiral galaxies (e.g in NGC 7793, NGC 300, NGC 4449, NGC 1705), according to multiple papers by Torsten Boker and colleagues (a group who have been working this problem for a long time) have been thought to be about the same size as standard globular clusters, though the most luminous NSCs have a substantially greater luminosity than even the brightest "standard old" globular cluster. (Some particularly luminous NSC have an impressive absolute V magnitude of minus 15). Also, an NSC can very obviously contain multiple populations of stars which were formed at various times, as compared to a globular, which usually (though not always!) contains a single population of stars that formed at the same time.

BUT>>>>>
there also exist star-cluster-like objects that can get larger than even the largest known globular star cluster: :
(1) Ultra Compact Dwarf (UCD) galaxies
(2) Nuclei of elliptical galaxies
( technically speaking, the radius of some of these objects is well outside of the usual range of effective radii that is occupied by the population of globular star clusters)

A particularly prominent example of an UCD galaxy was imaged by IIS member Rolf:
http://www.iceinspace.com.au/forum/showthread.php?t=92183&page=3

Renato1
07-06-2014, 02:23 AM
Thanks for finding all this fascinating stuff Robert.

Any idea how to actually get the paper for free discussed in the abstract in Item 5?

Are there any UCDs that can be glimpsed with an 8 or 14" telescope?

If there are all these NSCs in the middle of other spiral galaxies, I wonder how come they are all hidden in ours (assuming they are different in nature from the bluish, young open clusters that look like globular clusters that are to be found around the Magellanic Clouds).
Regards,
Renato

madbadgalaxyman
07-06-2014, 09:03 AM
You can get the draft of the paper by clicking on "arxiv preprint" and then downloading the pdf after another webpage has come up.
For papers older than 'sometime in 2012', it is often possible to get the final published paper by clicking on
'' full refereed journal article"

Your question about Ultra Compact Dwarf galaxies and Nuclear Star Clusters and supermassive young star clusters, and whether or not any of these objects can be glimpsed in the amateur telescope, is an interesting one. I will see if I can find the necessary information.
I believe that the two brightest star clusters in the nearby starburst Dwarf galaxy NGC 1569 have actually been seen by visual observers.

The nuclei of dwarf elliptical galaxies are also another 'possible' for amateur observation.

Mostly, extragalactic 'standard old globulars' are disappointingly 'out of reach' for visual observation, anywhere outside of the Local Group of Galaxies (= MW + M31 plus hangers on), with the possible exception of the brightest globulars of NGC 5128;
however, the most luminous Ultra Compact Dwarf galaxies and Nuclear Star Clusters can be several mags more luminous than even the biggest globular cluster.

The visual magnitude of the Ultra Compact Dwarf galaxy associated with the Sombrero is 17.6, which is still largely out of reach for visual obs. with standard amateur telescopes.

Incidentally, it now seems that the UCDs are probably just over-extended and exceedingly massive star clusters.

Q.
your question on visibility of nuclear star clusters in big Sbc and Sc spirals like the Milky Way Galaxy.
A.
High mass galaxies like the Milky Way tend to have a large population of interstellar dust grains that scatter and absorb the light of objects behind them, so it can be very hard to see nuclear star clusters in them;

even at the most favourable orientation for reducing extinction of light by interstellar dust (a face-on spiral), there can be two or more magns of extinction in front of a nuclear star cluster in one of these giant galaxies.
Therefore, NSCs are more easily viewed in smaller galaxies, because they have much less obscuring dust in their interstellar medium. Galaxies like NGC 7793 and NGC 300 and NGC 4449 are a couple of magns down in luminosity from the MW, and they are much less dusty.....therefore the NSCs are much easier to see in them.

Paddy
07-06-2014, 07:29 PM
Thanks for the links Robert and the interesting discussion. Hopefully I might be able to understand a little of the content of the papers.

madbadgalaxyman
08-06-2014, 03:26 PM
Here is yet another paper about the star clusters in M83!

It was just now published in the following reference:
(2014), Astrophysical Journal, Volume 787, page 17

The preprint of this paper can be obtained at this webpage:
http://arxiv.org/abs/1406.1157

I strongly suspect that the most luminous of the young star clusters in M83 may actually be glimpsed in visual observations with amateur telescopes;

Thim and Tammann and Saha and Sandage, in 2003, used Cepheid Variables to derive a dereddened distance modulus for M83 of 28.25, implying that this galaxy is at a Distance of 4.5 Megaparsecs (= 14.7 million light years)

Applying this Distance and Distance Modulus correction to an overluminous Young star cluster in M83 with a luminosity of Absolute Visual Magnitude minus 13 .........
implies an apparent visual magnitude of 15.25 for such a very luminous star cluster!

Added in edit:
The star cluster R136 (in the LMC), which is the most extreme young star cluster that has so far been discovered in the Milky Way and the Magellanic clouds, has a luminosity of Absolute Visual Magnitude Minus 11.7, and it weighs in at very roughly 60,000 solar masses.
This star cluster would be near to visual magnitude 16.5 if it were in M83, and, statistically speaking, it is likely that M83 (a much bigger galaxy than LMC) will have a few clusters that are brighter than R136

The galaxy M31 has a young star cluster known as VdB0 that is absolute visual magnitude minus 10.0 (probably the most luminous Young cluster known in the Andromeda Galaxy), and if I assume the modern compromise distance estimate for M31 from multiple measurement techniques (m-M = 24.4 and 760 kiloparsecs), this cluster could be brighter than 15 magn. in the telescope!

The reference arxiv 0812.1668 gives a measured V magnitude of 14.67 for VdB0, so it seems that I am not too far off in my reasoning.....