Halo stars in NGC 5128. II. An outer halo field and a new metallicity distribution

We present new Hubble Space Telescope WFPC2 (V, I) photometry for stars in NGC 5128 at a projected distance more than 30 kpc from the galaxy center. The color-magnitude diagram shows an old red-giant branch which is extremely broad in color, identical in its characteristics with our previously observed 20 kpc halo field (Harris et al.). To derive the metallicity distribution function (MDF) from the red giant color distribution, we use an extensive new grid of low-mass stellar models (VandenBerg et al.) calibrated against Milky Way globular clusters, along with a new interpolation procedure. We find that the halo of NGC 5128 is predominantly populated by moderately metal-rich stars in the range -1.0 < [m/H] < 0.0. The mean abundance is <Z > = 0.37 Z. (<m/H > = -0.43) with no detectable radial gradients. Since only 10% of the stars are more metal-poor than [m/H] similar or equal to -1, the MDF places severe restrictions on the fraction of the halo that could have been accreted from dwarf elliptical or small disk galaxies. A first-order chemical enrichment model which fits our observed MDF remarkably well is one in which the galaxy experienced two fairly distinct stages of halo formation: a very early "accreting-box" stage starting from primordial gas in which stars are already forming while the galaxy is still assembling by gas infall; and a second, more major stage of "closed-box" evolution dominated by star formation with little gas infall.