The properties of Galactic globular cluster sub-systems

Abstract

In this paper we compare the properties of three sub-systems of Galactic globular clusters, which are defined according to metallicity and HB morphology. We focus on cluster luminosities, structures, surface brightnesses (SB) and ellipticities. It is shown that the so-called "young" halo (YH) clusters, which may have been formed in external satellite galaxies, exhibit characteristics clearly distinct from those of the "old" halo (OH) and bulge/disk (BD) clusters, the majority of which are believed to be Galactic natives. The YH clusters have properties most similar to those of clusters belonging to several present day satellite dwarf galaxies. The OH and BD populations have apparently been modified by destructive tidal forces in the inner Galaxy. We estimate that the present population of Galactic natives may only represent some two-thirds of the original population. Several low SB clusters are highly flattened. We briefly speculate that this ellipticity may represent the intrinsic flattening of dark matter mini-halos in which such optically dim clusters might be embedded. Finally, we examine the distribution of clusters on the size vs.luminosity plane. Three objects fall above the sharp upper envelope of the main distribution on this plane: Omega Cen, M54, and NGC 2419. All three have previously, and independently, been suggested to be the stripped cores of former dwarf galaxies. This suspicion is strengthened by the observation that the massive cluster G1 in M31, plus a number of the most luminous clusters in NGC 5128 also fall in the same region of the plane. All of these objects have also previously been suggested to be the stripped cores of now-defunct dwarf galaxies.