Stellar halos in Illustris: probing the histories of Milky Way-mass galaxies

The existence of stellar halos around galaxies is a natural prediction of the hierarchical nature of the Lambda CDM model. Recent observations of Milky Way-like galaxies have revealed a wide range in stellar halo mass, including cases with no significant detection of a stellar halo, as in the case of M101, NGC 3351 and NGC 1042. We use the Illustris simulation to investigate the scatter in stellar halo content and, in particular, to study the formation of galaxies in the range M-200 = 8 x 10(11)-2 x 10(12) M-circle dot with the smallest fraction of this diffuse component. Stellar halos are far from spherical, which diminishes the surface brightness of the stellar halo for face-on disks. Once accounting for projection effects, we find that the stellar halo fraction f(SH) correlates strongly with galaxy morphology and star formation rate, but not with environment, in agreement with observations. Galaxies with the lowest stellar halo fractions are disk-dominated, star-forming, and assemble their dark matter halos earlier than galaxies with similar masses. They have also accreted more low-mass satellites at earlier infall times than centrals with high f(SH). In situ rather than accreted stars dominate the stellar halos of galaxies with the lowest stellar halo fractions, with a transition radius from in situ to accretion-dominated r similar to 45 kpc. Our results extrapolated to real galaxies such as M101 may indicate that these galaxies inhabit old halos which endured mergers at higher redshifts and evolved unperturbed in the last similar to 10 Gyrs.