Abstract
We use the Illustris simulations to gain insight into the build-up of the outer, low-surface brightness regions which surround galaxies. We characterize the stellar haloes by means of the logarithmic slope of the spherically averaged stellar density profiles, αSTARS at z = 0, and we relate these slopes to the properties of the underlying dark matter (DM) haloes, their central galaxies, and their assembly histories. We analyse a sample of ∼5000 galaxies resolved with more than 5 × 104 particles each, and spanning a variety of morphologies and halo masses (3 × 1011 ≤ Mvir ≲ 1014 M⊙). We find a strong trend between stellar halo slope and total halo mass, where more massive objects have shallower stellar haloes than the less massive ones (−5.5 ± 0.5 < αSTARS < −3.5 ± 0.2 in the studied mass range). At fixed halo mass, we show that disc-like, blue, young, and more massive galaxies are surrounded by significantly steeper stellar haloes than elliptical, red, older, and less massive galaxies. Overall, the stellar density profiles fall off much more steeply than the underlying DM, and no clear trend holds between stellar slope and DM halo concentration. However, DM haloes which formed more recently, or which accreted larger fractions of stellar mass from infalling satellites, exhibit shallower stellar haloes than their older analogues with similar masses, by up to ΔαSTARS ∼ 0.5–0.7. Our findings, combined with the most recent measurements of the strikingly different stellar power-law indices for M31 and the Milky Way, appear to favour a massive M31, and a Milky Way characterized by a much quieter accretion history over the past 10 Gyr than its companion.
Highlights
Observations of the Milky Way, M31, and other nearby galaxies demonstrate that the bright, central body of both early and late type galaxies is surrounded by an extended, faint envelope of stars (e.g. Martınez-Delgado et al 2010)
The paper is organised as follows: we introduce the adopted numerical simulations, methods and definitions in Section 2; our main result about the relationship between stellar halo slope, halo mass and galaxy properties is presented in Section 3; we compare the stellar and the DM density profiles in Section 4; in Section 5, we quantify the correlation between stellar halo slopes and the halo assembly histories, and we compare our findings to observations of the Milky Way and M31
While disk-like, star forming galaxies are rare at the highest mass end, their distribution on the ↵STARS MHalo diagram at fixed halo mass suggests that the stellar halo slope is connected in some way to the properties of the galaxy residing at its centre
Summary
Observations of the Milky Way, M31, and other nearby galaxies demonstrate that the bright, central body of both early and late type galaxies is surrounded by an extended, faint envelope of stars (e.g. Martınez-Delgado et al 2010). Numerical studies in this context have come mostly in two flavors: 1) via a combination of N-body-only simulations with semi-analytic models and/or stellar tagging techniques to mimic the stellar components (Cooper et al 2010; Rashkov et al 2012; Cooper et al 2013); 2) or via gravity+gasdynamics simulations of individual highly-resolved galaxies in cosmological context (Abadi, Navarro & Steinmetz 2006; Zolotov et al 2009; Tissera, White & Scannapieco 2012; Tissera et al 2013, 2014; Puchwein et al 2010) While the former cannot fully capture the differences in the orbital contents of stars and DM, and cannot reproduce the effects that baryonic physics might imprint into the underlying DM distribution (see Bailin et al 2014), the latter cannot assess the relevance of their outcome against the possibility of large halo-to-halo variations because of a lack of statistics.
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