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 (log Rh) 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 log Rh versus MV plane. All of the latter objects have previously been suggested as the stripped cores of now defunct dwarf galaxies.

The normal chemistry of multiple stellar populations in the dense globular cluster NGC 6093 (M 80)

“…than, inside, and redder than the instability strip, Lee 1990) is 0.93, 0.98, and −0.06 for NGC 288, M 80, and M 4, respectively (Mackey & van den Bergh 2005), corresponding to the differences clearly visible in the CMDs of the lower panels in Fig. 14. The HB stellar distribution in the first two GCs is all to the blue of the RR Lyrae instability strip, starting almost at the same colour, but reaching far more fainter (hotter) levels in M 80 than in NGC 288.…”

Homogeneous metallicities and radial velocities for Galactic globular clusters

“…The oldest isochrone gives the age of clusters at R < 8 kpc (Rey et al 2001). Pal 7 is not present in Mackey & van den Bergh (2005), so it is not plotted here. The arrows connect the position of the cluster if another [Fe/H] estimate is used, to the position determined by our metallicity scale.…”

“…13. The metallicity of our program clusters on the C09 scale is plotted here against HB-type from Mackey & van den Bergh (2005). Isochrones are from Rey et al (2001), and are separated by 1.1 Gyr from top to bottom.…”

TheGaia-ESO Survey: Kinematics of seven Galactic globular clusters

“…According to a two-tailed Student's test, the probability that a Spearman rank correlation coefficient equal to or higher than the observed one (r S = 0.423) is produced by chance from uncorrelated quantities is P t = 3.0% (27 clusters), Harris 1996Harris (2010 . (c) Mackey & van den Bergh (2005). Meaning of columns: (1) Cluster name; (2) central radial velocity dispersion; (3) error on σ 0 ; (4) projected rotation amplitude; (5) error on A rot ; (6) mean iron abundance ratio; (7) HB morphology, where HBR = (B − R)/(B + V + R), where B is the number of stars bluer than the instability strip, R redder, and V the number of variables in the strip; (8) the integrated V magnitude; (9) the isophotal ellipticity = 1 − (b/a); (10) central luminosity density; (11) distance from the Galactic centre (kpc).…”