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
140 kpc 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
dex per dex 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 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.
