Globular Clusters

[Fr. Wayne]

Ever since the 1950's when Pope Pius XII hosted the Astronomers the question of the relationship between globular clusters and their host galaxies has fascinated me. Can you point me toward the latest updates on any co-relationships between GC and the properties of local galaxies? For instance, back in the 50's I heard the term "shepherding" as a theory of their relationship.

[Fr. Wayne]

Duncan Forbes (Swinburne)
Title: Globular Clusters in the Local Group
I will review the status of our knowledge about the ~700 globular clusters known in the Local Group. Starting with our own Galaxy and its satellites, I will then discuss highlights from other galaxies. Globular clusters will be used to infer the star formation and accretion history of their host galaxies

[Tim Thompson]

I am uncertain as to what you are looking for, because I do not understand what a "co-relationship" is supposed to be.

Globular clusters tend to be distributed approximately spherically, even around a flat, spiral galaxy like the Milky Way or M31. The globulars are mostly very old; in the case of the Milky Way, all but a few of the globulars hold the oldest stars in the galaxy, older than the disk or bulge stars. The obvious implication is that the globulars formed during the early stages of galaxy formation. Globulars follow orbits around the Milky Way that are much the same kind of orbits as are follwed by long period, Oort cloud comets in our solar system; they are highly eccentric, and at all inclinations, hence the roughly spherical distribution.

The observational part is "easy", where they are and how they move. But where globular clusters come from, how they form, is quite unclear. There are basically two schools of though (and maybe two real, independent paths to globular cluster formation). One is that globulars condensed independently, early as single clusters, early on, as the Milky Way was just forming. The other is that globular clusters are the remnants of the condensed cores of dwarf spheriodal galaxies that have been gobbled up by the Milky Way. Today, I think it is a common theory that the very large globulars, such as Omega Centauri, or the G1 cluster at M31, are the remains of dwarf spheroidal galaxies, while the large majority of the smaller globulars are indendent clusters.

Now, maybe you can refer to what I have said here, to give me a better idea of what you are looking for? Meanwhile, here are a few examples from the recent scientific literature that look relevant; you can access PDF preprints of all of them. By following the "citations to the article" link, you can find other, more recent papers, which make reference to these, and may also be relevant.

• Kinematics of globular cluster systems and the formation of early-type galaxies; K. Bekki, et al., Monthly Notices of the Royal Astronomical Society, 363(4): 1211-1222, November 2005
• Extragalactic Globular Clusters: Old Spectroscopic Ages and New Views on Their Formation; Jay Strader, et al., Astronomical Journal 130(4): 1315-1323, October 2005
• New light on the formation and evolution of M31 and its globular cluster system; Puzia, Perrett & Bridges, Astronomy and Astrophysics 434(3): 909-934, May II 2005
• Globular Cluster and Galaxy Formation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies; David Burstein, et al., The Astrophysical Journal 614(1): 158-166, October 2004
• The elliptical galaxy formerly known as the Local Group: merging the globular cluster systems; Duncan Forbes, et al., Astronomy and Astrophysics 358: 471-480, June 2000

[Fr. Wayne]

Just reviewed your excellent summary- will study these links-and hopefully reply after Christmas. Thanks so much. I am indebted.

[Fr. Wayne]

Globular Clusters can be determined by age, color/metallicity, etc. To date the more we study GC in other galaxies, the more variety of types we discover. So far we can not determine for sure if GC are early progenitors of galaxies, accumulated remnants of "merged" galaxies, or just a overly-friendly stars which flocked together during periodic waves of increased star formation in the past. I realize this is a bit over-simplified for mass consumption but is this brief summary by a layman fair enough for starts?

[Tim Thompson]

My major critique would be regarding this: " ... just a overly-friendly stars which flocked together during periodic waves of increased star formation in the past." We can be pretty sure that is not the case, and that globular cluster stars are "coeval", meaning that they all have the same age, within the observational uncertainties. We know that because the GC color-magnitude diagrams show distinct main sequences, the ages of which can be determined from the main sequence turn-off point.

I would also avoid " ... the more variety of types ..." as being too vague. In fact, the "variety" is exclusively limited to differences in size (mass or linear diameter), and in age, so far as I know. We do find more young globular clusters than perhaps previously expected.

[Fr. Wayne]

I would also avoid " ... the more variety of types ..." as being too vague. In fact, the "variety" is exclusively limited to differences in size (mass or linear diameter), and in age, so far as I know. We do find more young globular clusters than perhaps previously expected.

MPC (metal-poor) and MRC (metal-rich) are then just age varieties? Let me add that I do appreciate your kind response. As you can guess, by flocking together, I had in mind the cliche "Birds of a feather...etc." but precise definitons as "coeval" are better if understood.

[Tim Thompson]

MPC (metal-poor) and MRC (metal-rich) are then just age varieties?

As far as I know, yes. The MPCs are older than the MRCs. The "metals" (remember that in astronomer jargon, anything heavier than helium is a metal) are created in stars, by stellar nucleosynthesis. The more time you have for more generations of stars to go by, the more metals you make. The older GCs will grow out of less processed material, and therefore be relatively metal poor.

[Fr. Wayne]

Evidently in the Milky Way only MPC's surround it, laying claim to a 10-12Gyr formation time. Can we infer that no dwarf galaxies have visited ours in that time period? If so, are globular clusters sentinels in that they record galactic collisions?

[Tim Thompson]

Evidently in the Milky Way only MPC's surround it, laying claim to a 10-12Gyr formation time. Can we infer that no dwarf galaxies have visited ours in that time period? If so, are globular clusters sentinels in that they record galactic collisions?

See my comments, and follow the links, in my 2nd paragraph, post #3. How globular culsters formed remains a topic actively under study. The larger GCs could be the condensed cores of dwarf galaxies (maybe, maybe not). While it certainly appears that the Milky Way GCs are very old, we do not yet know how they formed. If they are not the remnants of dwarf galaxies, then they do not record galaxy collisions.

[Fr. Wayne]

Fascinating! Maybe we'll someday see one in the process of forming.

[Nereid]

Evidently in the Milky Way only MPC's surround it, laying claim to a 10-12Gyr formation time. Can we infer that no dwarf galaxies have visited ours in that time period? If so, are globular clusters sentinels in that they record galactic collisions?

The Milky Way has been visited by a number of dwarf galaxies in this time (or, more accurately, some dwarfs have had the misfortune of coming too close to the MW), and we know of at least two that are in the process of being shredded - SagDEG and the Canis Major Dwarf.

SDSS has uncovered a number of interesting star streams, which may be the shredded remains of more impressive objects, which came off the worse in a tussle with the MW.

[Fr. Wayne]

Nereid: after reading your references I happened upon http://antwrp.gsfc.nasa.gov/apod/astropix.html This pix has an excellent view of a stage of globular cluster formations.

[Tim Thompson]

Nereid: after reading your references I happened upon http://antwrp.gsfc.nasa.gov/apod/astropix.html This pix has an excellent view of a stage of globular cluster formations.

I am guessing you meant this one: Arp 188 and the Tadpole's Tidal Tail. The link you posted is the one that changes every day. Select the archive link on that page and you get a list of all the images, back to 1995.

[Fr. Wayne]

I am guessing you meant this one: Arp 188 and the Tadpole's Tidal Tail. The link you posted is the one that changes every day. Select the archive link on that page and you get a list of all the images, back to 1995.

Thanks. I will be more precise about links. Still a week from now when someone clicks on it, they might really try to understand the connection and solve all the mysteries of the Universe by accident. Serendity- doo!