A Color - Magnitude Diagram for a Globular Cluster in the Giant Elliptical Galaxy NGC 5128

The Hubble Space Telescope has been used to obtain WFPC2 (V, I) photometry for a large sample of stars in the outer halo of the giant elliptical NGC 5128, at a distance 4 Mpc beyond the Local Group. The globular cluster N5128-C44, at the center of the Planetary Camera field, is well enough resolved to permit the construction of a color-magnitude diagram (CMD) for it which covers the brightest two magnitudes of the giant branch. The CMD is consistent with that of a normal old, moderately low-metallicity ([Fe/H] ≃ −1.3) globular cluster, distinctly more metal-poor than most of the field halo stars at the same projected location (which average [Fe/H] ∼ −0.5). This is the most distant globular cluster in which direct color-magnitude photometry has been achieved to date, and the first one belonging to a giant E galaxy.