Terzan 5's interesting past

[ngcles]

Hi All,

For those with an interest in globular clusters, I found this the other day on Spaceref and went to the ESO page and found a little more detail there:

http://www.eso.org/public/outreach/press-rel/pr-2009/pr-45-09.html

It seems the globular cluster Terzan 5 might not be what we thought it was all these years (a classical GC) but is instead likely the remnant core of a dwarf galaxy that long, long ago merged with the Milky Way Galaxy.

Omega Centauri and M54 are also strongly suspected of being in this category for similar reasons.

Interesting.


Best,

Les D

[Robh]

Les. Very interesting.

There is also speculation that both omega Centauri and M54 have a central intermediate-mass black hole. Does this lend more support to the theory they may have been the cores of a dwarf galaxy?

Regards, Rob

[renormalised]

Yes it does. They suspect that most "normal" globulars haven't had sufficient time, and in most cases sufficient mass, to have formed an intermediate mass black hole in their cores.

[ngcles]

Hi Rob & Carl,



It would seem entirely sensible to look for mid-mass blackholes in the largest G.C. I'm not certain but I think they may have already ruled out Omega Centauri (NGC 5139). I'm not sure whether a mid-mass black hole would make it more or less likely a "left-over" -- most dwarf galaxies don't really have much of a core and without that not terribly likely to harbor a hole.

Not sure about M54 (seems a logical place to look though). These are the Milky Way's most massive two clusters and both appear to be remnant cores of long ago devoured snacks.

Omega Cen certainly has had more than one round of star-birth. From memory there are two if not three distinct bands of metallicity in the cluster implying three seperate epochs of star-forming which would be abnormal for a classical G.C.

The leading candidate for a mid-mass black hole I think (from memory) is M15 -- a classical G.C. There is at least one paper that comes down heavily to that conclusion. A couple of other papers have cast doubt on that. It would probably be dangerous to conclude a hole based only on the movement of stars in the core -- you need some other empirical evidence I'd think at least as some form of corroboration.

The core of M15 (the Milky Way's most compressed G.C) is a very extreme place. The average distance between stars in the central few parsecs is in the order of 100 A.U. Due to the dynamics of core colapse and mass segregation (M15 has undergone extreme core-collapse), in this area there are likely to be hundreds if not thousands of "heavy" white dwarfs and dozens if not hundreds of neutron stars as well as ordinary stars. But they are too faint to see or resolve, so we don't know the exact number with any great certainty.

That being so, if would be difficult to conclude that the central object is a hole, rather than say a compressed, concentrated population of stellar-remnants (white dwarfs, neutron stars) that mass-wise, gravitationally imitates the effect of a mid-mass black hole.

Terzan 5 being in the same catagory as M54 and Omega Centauri came as somewhat of a suprise I'd reckon because it is not a particularly massive or luminous G.C -- even after this recent "upgrade".

But I guess it just goes to show that the somewhere, somewhen, the Universe is always prepared to leap-out from behind a rock somewhere and say "boo" when you least expect it ...


Best,

Les D

[Lismore Bloke]

Hi everyone, hope I'm not butting in, but I just remembered the Wikipedia article on Omega. It touches on some of the topics mentioned. The links in the article are very interesting - the article is where I first heard about the possibility that some globulars are highly likely to be the core remnants of small galaxies swallowed by the milky way. Fascinating stuff.


http://en.wikipedia.org/wiki/Omega_Centauri

[gary]

Thanks for the article link Les. Extremely interesting study. Another good example
of when one galaxy gets mixed up with another, there is always the chance that one of them might get their galactic heart ripped out!