The SLUGGS survey: Combining stellar and globular cluster metallicities in the outer regions of early-type galaxies

TL;DR

This study measures stellar and globular cluster metallicities in the outer regions of 12 early-type galaxies, revealing similar gradients for different globular cluster populations and insights into galaxy assembly histories.

Contribution

It provides new measurements of metallicity gradients in galaxy outskirts, challenging existing theories and linking globular cluster properties to galaxy mass and assembly processes.

Findings

Similar metallicity gradients for metal-poor and metal-rich globular clusters.

Steeper globular cluster metallicity gradients in lower-mass galaxies.

Outer galaxy stars and globular clusters show correlated metallicity trends.

Abstract

The outer halo regions of early-type galaxies carry key information about their past accretion history. However, spectroscopically probing the stellar component at such galactocentric radii is still challenging. Using Keck/DEIMOS, we have been able to measure the metallicities of the stellar and globular cluster components in 12 early-type galaxies out to more than $10~\rm{R_{e}}$. We find similar metallicity gradients for the metal-poor and metal-rich globular cluster subpopulations, suggesting a common formation process for the two subpopulations. This is in conflict with most current theoretical predictions, where the metal-poor globular clusters are thought to be purely accreted and metal-rich globular clusters mostly formed in-situ. Moreover, we find that the globular cluster metallicity gradients show a trend with galaxy mass, being steeper in lower-mass galaxies than in higher-mass galaxies. This is similar to what we find for the outermost galaxy stars and suggests a more active accretion history, with a larger role played by major mergers, in the most massive galaxies. This conclusion is qualitatively consistent with expectations from two-phase galaxy assembly models. Finally, we find that the small difference in metallicity between galaxy stars and metal-rich globular clusters at $1~\rm{R_{e}}$ may correlate with galaxy mass. The origin of this difference is not currently clear.