Metallicity gradients in the globular cluster systems of early-type galaxies: in situ and accreted components?

Massive early-type galaxies typically have two subpopulations of globular clusters (GCs) which often reveal radial colour (metallicity) gradients. Collating gradients from the literature, we show that the gradients in the metal-rich and metal-poor GC subpopulations are the same, within measurement uncertainties, in a given galaxy. Furthermore, these GC gradients are similar in strength to the stellar metallicity gradient of the host galaxy. At the very largest radii (e.g. greater than 8 galaxy effective radii), there is some evidence that the GC gradients become flat with near constant mean metallicity. Using stellar metallicity gradients as a proxy, we probe the assembly histories of massive early-type galaxies with hydrodynamical simulations from the Magneticum suite of models. In particular, we measure the stellar metallicity gradient for the in situ and accreted components over a similar radial range as those observed for GC subpopulations. We find that the in situ and accreted stellar metallicity gradients are similar but have a larger scatter than the metal-rich and metal-poor GC subpopulations' gradients in a given galaxy. We conclude that although metal-rich GCs are predominately formed during the in situ phase and metal-poor GCs during the accretion phase of massive galaxy formation, they do not have a strict one-to-one connection.