Simultaneous modelling of the stellar halo and globular cluster system of NGC 5128

An important test for models of galaxy formation lies in the metallicity distribution functions (MDFs) of spheroid stars and their globular clusters (GCs). We have compared the MDFs obtained from spectroscopy of the GCs and the star-by-star photometry of the old halo red giants in the nearby elliptical galaxy NGC 5128, with the predictions of a Λ-cold dark matter (ΛCDM) semi-analytic galaxy formation model. We have selected model ellipticals comparable in luminosity and environment to NGC 5128, and reconstructed their MDFs by summing the total star formation occurring over all their progenitors. A direct comparison between models and data shows that the MDFs are qualitatively similar, both have stellar components that are predominantly metal-rich (∼0.8 Z⊙), with a small fraction of metal-poor stars extending down to 0.002 Z⊙. The model MDFs show only small variations between systems, whether they constitute brightest cluster galaxies or low-luminosity group ellipticals. Our comparison also reveals that these model MDFs harbour a greater fraction of stars at Z > Z⊙ than the observations, producing generally more metal-rich (by ∼0.1 dex) MDFs. One possibility is that the outer-bulge observations are missing some of the highest-metallicity stars in this galaxy. We find good agreement between the model and observed GC MDFs, provided that the metal-poor GC formation is halted early (z∼ 5) in the model. Under this proviso, both the models and data are bimodal with peaks at 0.1 Z⊙ and Z⊙, and cover similar metallicity ranges. This broad agreement for the stars and GCs suggests that the bulk of the stellar population in NGC 5128 may have been built up in a hierarchical fashion, involving both quiescent and merger-induced star formation. The predicted existence of age structure amongst the metal-rich GCs needs to be tested against high-quality data for this galaxy.