Evidence for two phases of galaxy formation from radial trends in the globular cluster system of NGC 1407

Here we present the colours of individual globular clusters (GCs) around the massive elliptical galaxy NGC 1407 out to a projected galactocentric radius of 140kpc or 17 galaxy effective radii (R_e). Such data are a proxy for the halo metallicity. We find steep, and similar, metallicity gradients of ∼-0.4 dexdex^-1 for both the blue (metal poor) and red (metal rich) GC subpopulations within 5-8.5 R_e (40-70 kpc). At larger radii the mean GC colours (metallicity) are constant. A similar behaviour is seen in a wide-field study of M87's GC system, and in our own Galaxy. We interpret these radial metallicity trends to indicate an inner region formed by early in situ dissipative processes and an outer halo formed by the ongoing accretion of low-mass galaxies and their GCs. These results provide observational support for the model of galaxy formation whereby massive galaxies form inside-out in two phases. We have also searched the literature for other massive early-type galaxies with reported GC metallicity gradients in their inner regions. No obvious correlation with galaxy mass or environment is found but the sample is currently small.