Origin of the metallicity distribution of the NGC 5128 stellar halo

Recent Hubble Space Telescope photometry in the nearby elliptical galaxy NGC 5128 shows that its halo field star population is dominated by moderately metal-rich stars, with a peak at [m/H]≃−0.4 and with a very small fraction of metal-poor ([m/H] < −1.0) stars. In order to investigate the physical processes which may have produced this metallicity distribution function (MDF), we consider a model in which NGC 5128 is formed by the merging of two major spiral galaxies. We find that the halo of an elliptical formed in this way is predominantly populated by moderately metal-rich stars with [m/H]∼−0.4, which were initially within the outer parts of the two merging discs and were tidally stripped during the merger. To match the NGC 5128 data, we find that the progenitor spiral discs must have rather steep metallicity gradients similar to that defined by the Milky Way open clusters, as well as sparse metal-poor haloes (5 per cent or less of the disc mass). Very few stars from the central bulges of the spiral galaxies end up in the halo, so the results are not sensitive to the relative sizes (bulge-to-disc ratios) or metallicities of the initial bulges. Finally, we discuss the effects on the globular cluster (GC) system. The emergent elliptical will end up with metal-poor halo clusters from the original spiral haloes, but with moderately metal-rich halo stars from the progenitor discs, thus creating a mean offset between the MDFs of the halo stars and the GC system. Remaining questions yet to be answered concern the total size of the GC system population (the ‘specific frequency problem’) and the observed existence of metal-rich GCs in large numbers in the outer haloes of giant ellipticals. We also discuss possible differences in the MDFs of stellar haloes of galaxies of different Hubble type.