Hi All,
As _I understand_ it, may be a few of the GCs that are indeed remnant cores from small galaxies gobbled up by the Milky Way (Omega Centauri NGC 5139 might well fall into this category), but the vast majority are not.
The GCs are all uniformly ancient stars -- among the very first stars born within the primordial Milky Way. Way, way back in the mists of time, when it was collapsing from its roughly spherical gas cloud, but before most of the material had collapsed to the disc, the first stars were born into vast, vast clusters -- in some cases more than a million stars. All the high-mass stars in these clusters evolved quickly and exploded as supernovae leaving behind only the low-mass stars we see today -- about 10gyr on. The stars of the GCs are different from the general population of stars we see today; they are in effect fossils. Because they came out of the material of the big-bang they contain virtually no metals (ie material heavier than 2He). Because they were not born of the disc, the globulars do not inhabit the disc but are arranged in a more-or-less spherical halo around the core.
Gravity (the same force that sticks us to the ground) (Alex excepted
) is what holds 'em together. With those clusters that pass through or close to our core, they do undergo stripping and loose stars that usually end-up populating the halo. Two examples of clusters that have either undergone or are undergoing stripping are Pal 5 and E3 -- the latter is likely our galaxy's least massive and least luminous GC. No doubt over time, some of the Milky Way GCs have been completely evaporated in this way.
The Milky Way's original retinue of clusters have likely been added to significantly by the Milky Way gobbling up little galaxies and stealing their clusters. For example, the Sagittarius Dwarf Elliptical that is currently undergoing dismemberment is likely in the process to be donating several more GCs -- from memory, these include M54 (which _might_ be the core of that galaxy) Rup 106, Pal 12, Arp GC 2 and maybe a couple of others. These "donated" GCs can be inferred from their distances, motions and (slightly different) metalicity.
Similarly, the recently discovered (still disputed though) Canis Major Dwarf _may_ have donated M79, NGC 1851 and NGC 2298 that are all, suspiciously enough, located at the roughly the same distance from the core of the Milky Way a long, long way out from the core (where you probably wouldn't expect to find 1, let alone 3) and quite near (relatively) each other. There is no other evidence at this stage to support that hypothesis. Richard Lane who works at Sydney Observatory is currently doing a lot of research on the alleged Canis Major Dwarf in order to prove/disprove its existence.
The reason why Omega seems different from the "other" GCs, lies in the fact that its stars have low metalliciity (like a GC) but, there are three slightly but distinctly different bands of metallicity within its population inferring that some of its stars were born at different times -- which would be highly unusual for a true GC.
Best,
Les D
Conntributing Editor
AS&T