The information content of stellar halos: Stellar population gradients and accretion histories in early-type Illustris galaxies

TL;DR

This study uses the Illustris simulation to show that stellar population gradients in galaxy halos, especially metallicity and surface brightness, can reveal key aspects of galactic accretion histories, aiding future observational efforts.

Contribution

It demonstrates that stellar population gradients in early-type galaxy halos encode information about their accretion histories, with metallicity and surface-brightness profiles being particularly informative.

Findings

Galaxies with less accreted halo material have steeper metallicity profiles.

Metallicity and surface-brightness profiles tend to flatten from z=1 to present.

Age gradients are not strongly linked to accretion histories.

Abstract

Long dynamical timescales in the outskirts of galaxies preserve the information content of their accretion histories, for example in the form of stellar population gradients. We present a detailed analysis of the stellar halo properties of a statistically representative sample of early-type galaxies from the Illustris simulation and show that stellar population gradients at large radii can indeed be used to infer basic properties of galactic accretion histories. We measure metallicity, age, and surface-brightness profiles in quiescent Illustris galaxies ranging from $\mathrm{M}_\star = 10^{10} - 2\times 10^{12}\;\mathrm{M}_\odot$ and show that they are in reasonable agreement with observations. At fixed mass, galaxies that accreted little of their stellar halo material tend to have steeper metallicity and surface-brightness profiles between 2 - 4 effective radii (R$_e$) than those with larger accreted fractions. Profiles of metallicity and surface-brightness in the stellar halo typically flatten from z = 1 to the present. This suggests that the accretion of stars into the stellar halo tends to flatten metallicity and surface-brightness profiles, a picture which is supported by the tight correlation between the two gradients in the stellar halo. We find no statistical evidence of additional information content related to accretion histories in stellar halo metallicity profiles beyond what is contained in surface-brightness profiles. Age gradients in the stellar halo do not appear to be sensitive to galactic accretion histories, and none of the stellar population gradients studied are strongly correlated with the mean merger mass-ratio. Future observations that reach large radii outside galaxies will have the best potential to constrain galactic accretion histories.