Globular cluster subsystems in the Galaxy

Data from the literature are used to construct a homogeneous catalog of fun damental astrophysical parameters for 145 globular clusters of the Milky Wa y Galaxy. The catalog is used to analyze the relationships between chemical composition, horizontal-branch morphology, spatial location, orbital eleme nts, age, and other physical parameters of the clusters. The overall globul ar-cluster population is divided by a gap in the metallicity function at [F e/H] = -1.0 into two discrete groups with well-defined maxima at [Fe/H] = - 1.60 +/- 0.03 and -0.60 +/- 0.04, The mean spatial-kinematic parameters and their dispersions change abruptly when the metallicity crosses this bounda ry. Metal-poor clusters occupy a more or less spherical region and are conc entrated toward the Galactic center. Metal-rich clusters (the thick disk su bsystem), which are far fewer in number, are concentrated toward both the G alactic center and the Galactic plane. This subsystem rotates with an avera ge velocity of V-rot = 165 +/- 28 km/s and has a very steep negative vertic al metallicity gradient and a negligible radial gradient. It is, on average , the youngest group, and consists exclusively of clusters with extremely r ed horizontal branches. The population of spherical-subsystem clusters is a lso inhomogeneous and, in turn, breaks up into at least two groups accordin g to horizontal-branch morphology. Clusters with extremely blue horizontal branches occupy a spherical volume of radius similar to 9 kpc, have high ro tational velocities (V-rot = 77 +/- 33 km/s), have substantial and equal ne gative radial and vertical metallicity gradients, and are, on average, the oldest group (the old-halo subsystem). The vast majority of clusters with i ntermediate-type horizontal branches occupy a more or less spherical volume approximate to 18 kpc in radius, which is slightly flattened perpendicular to the Z direction and makes an angle of approximate to 30 degrees to the X-axis. On average, this population is somewhat younger than the old-halo c lusters (the young-halo subsystem), and exhibits approximately the same met allicity gradients as the old halo. As a result, since their Galactocentric distance and distance from the Galactic plane are the same, the young-halo clusters have metallicities that are, on average, Delta[Fe/H] approximate to 0.3 higher than those for old-halo clusters. The young-halo subsystem, w hich apparently consists of objects captured by the Galaxy at various times , contains many clusters with retrograde orbits, so that its rotational vel ocity is low and has large errors, V-rot = -23 +/- 54 km/s. Typical paramet ers are derived for all the subsystems, and the mean characteristics of the ir member globular clusters are determined. The thick disk has a different nature than both the old and young halos. A scenario for Galactic evolution is proposed based on the assumption that only the thick-disk and old-halo subsystems are genetically associated with the Galaxy. The age distribution s of these two subsystems do not overlap. It is argued that heavy-element e nrichment and the collapse of the proto-Galactic medium occurred mainly in the period between the formation of the old-halo and thick-disk subsystems. (C) 2000 MAIK "Nauka/Interperiodica".