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

Abstract

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 probes a mixture of its inner and outer bulge. The color-magnitude diagram shows an old red giant branch that is even broader in color than our two previously studied outer 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 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 spiral and elliptical galaxies. We find that the main features of the 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 fields we have studied there is a clear decrease of specific frequency SN (number of clusters per unit light) with increasing metallicity. At the lowest-metallicity range ([Fe/H] -1 it has dropped to 1.5. 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 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-SN population. The high specific frequency at low metallicity may therefore be related to the comparably large SN values found in the most metal-poor dwarf elliptical galaxies.