Stellar populations of stellar halos: Results from the Illustris simulation

TL;DR

This study uses the Illustris simulation to analyze stellar population gradients in quiescent galaxy halos, revealing their connection to galaxy assembly and merger history.

Contribution

It provides a theoretical framework linking stellar halo properties to galaxy build-up, based on a comprehensive simulation analysis.

Findings

Wide variance in stellar population gradients among similar-mass galaxies

Metallicity gradient correlates with in-situ star formation fraction

Results align with observed galaxy halo properties

Abstract

The influence of both major and minor mergers is expected to significantly affect gradients of stellar ages and metallicities in the outskirts of galaxies. Measurements of observed gradients are beginning to reach large radii in galaxies, but a theoretical framework for connecting the findings to a picture of galactic build-up is still in its infancy. We analyze stellar populations of a statistically representative sample of quiescent galaxies over a wide mass range from the Illustris simulation. We measure metallicity and age profiles in the stellar halos of quiescent Illustris galaxies ranging in stellar mass from $10^{10}$ to $10^{12} M_\odot$, accounting for observational projection and luminosity-weighting effects. We find wide variance in stellar population gradients between galaxies of similar mass, with typical gradients agreeing with observed galaxies. We show that, at fixed mass, the fraction of stars born in-situ within galaxies is correlated with the metallicity gradient in the halo, confirming that stellar halos contain unique information about the build-up and merger histories of galaxies.