Globular clusters and the formation of the outer Galactic halo

ABSTRACT Globular clusters in the outer halo (Rgc > 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 Rh < 10 pc are only found at Rgc < 40 kpc, while on the other hand the larger clusters with Rh > 10 pc are encountered at all Galactocentric distances. Among the compact clusters with Rh < 10 pc that have Rgc > 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 Large Magellanic Cloud, Small Magellanic Cloud 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 ω Centauri.