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

TL;DR

This study compares metallicity gradients in globular cluster subpopulations and stellar components of early-type galaxies, revealing similar gradients and complex formation histories involving in-situ and accreted processes.

Contribution

It provides a comprehensive analysis of GC and stellar metallicity gradients, combining literature data and hydrodynamical simulations to explore galaxy assembly mechanisms.

Findings

GC subpopulations have similar metallicity gradients within galaxies.

Gradients in GCs are comparable to those in the host galaxy's stars.

Metallicity gradients flatten at large radii (>8 effective radii).

Abstract

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 {\it 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.