Globular clusters and the formation of the outer Galactic halo

Sidney van den Bergh, Dougal Mackey

Published 2004-07-16Version 1

Globular clusters in the outer halo (R_gc > 15 kpc) are found to be systematically fainter than those at smaller Galactocentric distances. Within the outer halo the compact clusters with half-light radii R_h < 10 pc are only found at R_gc < 40 kpc, while on the other hand the larger clusters with R_h > 10 pc are encountered at all Galactocentric distances. Among the compact clusters with R_h < 10 pc that have R_gc > 15 kpc, there are two objects with surprisingly high metallicities. One of these is Terzan 7, which is a companion of the Sagittarius dwarf. The other is Palomar 1. The data on these two objects suggests that they might have had similar evolutionary histories. It is also noted that, with one exception, luminous globular clusters in the outer halo are all compact whereas faint ones may have any radius. This also holds for globular clusters in the LMC, SMC and Fornax dwarf. The lone exception is the large luminous globular NGC 2419. Possibly this object is not a normal globular cluster, but the stripped core of a former dwarf spheroidal. In this respect it may resemble Omega Centauri.