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 (d ≃ 4 Mpc). The target region is located at a projected distance 21 kpc from the center of NGC 5128. 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 that covers the brightest 2 mag of the giant branch. The CMD is consistent with that of a normal old, intermediate-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.