Globular Clusters in a Globular Cluster

We present one of the deepest optical images ever taken with the Hubble Space Telescope Advanced Camera for Surveys, a 126 orbit integration

of a small field within the nearby Galactic globular cluster NGC 6397. The data set allows us to construct a very clean, deep (V = 30)

color-magnitude diagram from which we study the stellar populations of this cluster (see Richer et al. abstract at this meeting). In analyzing the distribution of objects across the deep image of NGC 6397, we discovered an unexpectedly high concentration of several hundred faint, blue point-like sources in one region of the image. A closer look revealed the presence of a large background elliptical galaxy at the center of this distribution of unresolved sources. These objects are themselves extragalactic star clusters bound to this elliptical

galaxy. We measure a redshift for the galaxy using the Gemini/GMOS spectrograph and conclude that this is one of the furthest samples of star clusters ever studied and therefore could potentially hold

important clues for understanding the evolution of globular clusters.

This research was funded by grants from NASA/STScI.