The properties of Galactic globular cluster subsystems

Abstract

In this paper we compare the properties of three subsystems of Galactic globular clusters, which are defined according to metallicity and horizontal branch morphology. We specifically focus on cluster luminosities, structures, surface brightnesses and ellipticities. It is shown that the so-called ‘young’ halo (YH) clusters, which are thought to have formed in external satellite galaxies, exhibit characteristics which are clearly distinct from those of the ‘old’ halo (OH) and bulge/disc (BD) clusters, the majority of which are believed to be Galactic natives. The properties of the YH objects are, in many respects, similar to those of clusters belonging to a number of present-day satellite dwarf galaxies. The OH and BD populations have apparently been strongly modified by destructive tidal forces and shocks in the inner Galaxy. By comparing the properties of the three cluster subsystems, we estimate that the present population of native Galactic clusters may only represent approximately two-thirds of the original population. Several clusters with low surface brightnesses are observed to be highly flattened. We briefly speculate on the possibility that this ellipticity reflects the intrinsic flattening of dark matter mini-haloes in which these optically dim clusters might be embedded. Finally, we examine the distribution of clusters on the size (logRh) versus luminosity (MV) plane. Three objects are seen to fall well above the sharp upper envelope of the main distribution of clusters on the size–luminosity plane: ω Centauri, M54 and NGC 2419. All three of these objects have previously, and independently, been suggested to be the stripped cores of former dwarf galaxies. This suspicion is strengthened by the additional 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 logRh versus MV plane. All of the latter objects have previously been suggested as the stripped cores of now defunct dwarf galaxies.