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 ] \simeq -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 merging of two major spiral galaxies .
We find that the halo of an elliptical formed this way is predominantly populated by moderately metal-rich stars with [ m/H ] \sim -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 the one defined by the Milky Way open clusters , as well as sparse metal-poor haloes ( 5 % 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 system ( GCS ) .
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 GCS .
Remaining questions yet to be answered concern the total size of the GCS population ( the “ specific frequency problem ” ) and the observed existence of metal-rich globular clusters 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 .