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We use the Illustris Simulations to gain insight into the build-up of theouter, low-surface brightness regions which surround galaxies. We characterizethe stellar haloes by means of the logarithmic slope of thespherically-averaged stellar density profiles, alphaSTARS at z=0, and we relatethese slopes to the properties of the underlying Dark-Matter (DM) haloes, theircentral galaxies, and their assembly histories. We analyze a sample of ~5,000galaxies resolved with more than 5x10^4 particles each, and spanning a varietyof morphologies and halo masses (3x10^11 < Mvir < 10^14 Msun). We find a strongtrend between stellar halo slope and total halo mass, where more massiveobjects have shallower stellar haloes than the less massive ones (-5.5 \pm 0.5< alphaSTARS <-3.5 \pm 0.2 in the studied mass range). At fixed halo mass, weshow that disk-like, blue, young, and more massive galaxies are surrounded bysignificantly steeper stellar haloes than elliptical, red, older, and lessmassive galaxies. Overall, the stellar density profiles fall off much moresteeply than the underlying DM, and no clear trend holds between stellar slopeand DM halo concentration. However, DM haloes which formed more recently, orwhich accreted larger fractions of stellar mass from infalling satellites,exhibit shallower stellar haloes than their older analogs with similar masses,by up to Delta(alphaSTARS) ~ 0.5-0.7. Our findings, combined with the mostrecent measurements of the strikingly different stellar power-law indexes forM31 and the Milky Way, appear to favour a massive M31, and a Milky Waycharacterized by a much quieter accretion history over the past 10 Gyrs thanits companion.

Halo Mass and Assembly History Exposed in the Faint Outskirts: the Stellar and Dark Matter Haloes of Illustris Galaxies