The Halo Stars in NGC 5128. III: An Inner-Halo Field and the Metallicity Distribution

We present new Hubble Space Telescope WFPC2 ( V, I ) photometry for field stars in NGC 5128 at a projected distance of 8 kpc from the galaxy center, which probe a mixture of its inner halo and outer bulge. The color-magnitude diagram shows an old red-giant branch which is even broader in color than our two previously studied outer-halo fields (at 21 and 31 kpc), with significant numbers of stars extending to Solar metallicity and higher. The peak frequency of the metallicity distribution function (MDF) is at [m/H] ≃ − 0 . 4, with even fewer metal-poor stars than in the outer-halo fields. If we use the 21- and 31-kpc fields to define template “halo” MDFs and subtract these from the 8-kpc field, the residual “bulge” population has a mean [m/H] ≃ − 0 . 2, similar to the bulges of other large spirals and ellipticals. We find that the main features of the halo MDF can be reproduced by a simple chemical evolution model in which early star formation goes on simultaneously with an initial stage of rapid infall of very metal-poor gas, after which the infall dies away exponentially. Finally, by comparison with the MDFs for the NGC 5128 globular clusters, we find that in all the halo fields we have studied, there is a clear decrease of specific frequency S N (number of clusters per unit halo light) with increasing metallicity. At the lowest-metallicity range ([Fe/H] < − 1 . 6) S N This trend may indicate that globular cluster formation efficiency is a strong function of the metallicity of the protocluster gas. However, we suggest an alternate possibility, which is that globular clusters form preferentially sooner than field stars. If most of the cluster formation within a host giant molecular cloud (GMC) takes place sooner than most of the distributed field-star formation, and if the earliest, most metal-poor star-forming clouds are prematurely disrupted by their own first bursts of star formation, then they would leave relatively few field stars with a high − S N population. The high specific frequency at low metallicity may therefore be related to the comparably large S N values found in the most metal-poor dwarf ellipticals.