Globular clusters in the outer galactic halo: AM-1 and palomar 14

Abstract

AM-1, at 120 kpc, and Palomar 14 (Pal 14), at 70 kpc, are two of the most distant Galactic globular clusters (GCs) known. We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry of AM-1 and Pal 14 that reveals unprecedented depth and detail in the color-magnitude diagrams of these two clusters. Absolute and relative age measurements confirm that both are younger than the inner halo GC M 3 by 1.5-2 Gyr assuming all three clusters have similar compositions. Thus AM-1 and Pal 14 join Pal 3, Pal 4, and Eridanus as distant GCs with red horizontal branches (HBs) and young ages relative to the inner halo. Within the context of the entire body of research on the ages of second parameter GCs, the observed correlation between age and HB morphology suggests that age is the best candidate for the second parameter. However, this conclusion is tempered by the lack of precise chemical abundance determinations for a significant fraction of second parameter GCs. © 2008. The American Astronomical Society. All rights reserved.