Abstract
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 = 1010–2 × 1012 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 ( ), 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. Our findings relate specifically to regions of the stellar halo within 4 , but suggest that future observations that reach large radii outside galaxies (including to 10 and beyond) will have the best potential to constrain galactic accretion histories.
Highlights
The ΛCDM model of cosmology makes strong predictions regarding the expansion of the universe and the hierarchical growth of structure on large scales that have been supported by numerous observations (Smoot et al 1992; Eisenstein et al 2005; Hinshaw et al 2013), but the formation and evolution of galaxies remain poorly understood
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 be used to infer basic properties of galactic accretion histories
We have investigated the observable stellar properties of simulated early-type galaxies, and how stellar population gradients are connected to the history of accretion in the stellar halo
Summary
The ΛCDM model of cosmology makes strong predictions regarding the expansion of the universe and the hierarchical growth of structure on large scales that have been supported by numerous observations (Smoot et al 1992; Eisenstein et al 2005; Hinshaw et al 2013), but the formation and evolution of galaxies remain poorly understood (see reviews in Conselice 2014; Somerville & Davé 2015). It is generally accepted that galaxies form through a combination of two complementary mechanisms: the in-situ formation of new stars, and the accretion of ex-situ stars via mergers (e.g., Kobayashi 2004; Oser et al 2010; Pillepich et al 2015; Rodriguez-Gomez et al 2016) These two formation channels can, for example, help explain the dramatic size evolution of massive early-type galaxies: extremely compact “red nuggets” are found at z ≈ 2 that are factors of 2 − 4 times smaller than z = 0 ETGs of similar masses (e.g., Daddi et al 2005; Trujillo et al 2006, 2007; van Dokkum et al 2008). The accumulated light at large radii around ETGs should contain important information about their accretion histories
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