Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch**Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO-9373. Also partially based on observations collected at the European Southern Observatory, La Silla, Chile, in Observing Programme 071.D-0560.

Abstract

Using the Hubble Space Telescope (HST) Wide Field Camera (WFC) of the Advanced Camera for Surveys (ACS), we have obtained deep (V, I) photometry of an outer halo field in NGC 5128, to a limiting magnitude of I ≃ 29. Our photometry directly reveals the core helium burning stellar population (the "red clump" or horizontal branch) in a giant E/S0 galaxy for the first time. The color-magnitude diagram displays a very wide red giant branch (RGB), an …