On the Formation of Galaxy Halos: Comparing NGC 5128 and the Local Group Members

The metallicity distribution function (MDF) for the old red giant stars in the halo of NGC 5128, the nearest giant elliptical galaxy, is virtually identical with the MDF for the old-disk stars in the LMC and also strongly resembles the halo MDF in M31. These galaxies all have high mean halo metallicities (⟨m/H⟩ ≃ -0.4) with very small proportions of low-metallicity stars. These observations reinforce the view that metal-rich halos are quite normal for large galaxies of all types. Such systems are unlikely to build up by accretion of preexisting gas-free small satellite galaxies, unless these satellites had an extremely shallow mass distribution (Δ log N/Δ log M≳ -1). We suggest that the halo of NGC 5128 is more likely to have assembled from hierarchical merging of gas-rich lumps in which the bulk of star formation took place during or after the merger stage.