This paper explores the mapping between the observable properties of a stellar halo in phase and abundance space and the parent galaxy's accretion history in terms of the characteristic epoch of accretion and mass and orbits of progenitor objects. The study utilizes a suite of 11 stellar halo models constructed within the context of a standard ÃCDM cosmology. The results demonstrate that coordinate-space studies are sensitive to the recent (0Y8 Gyr ago) merger histories of galaxies (this timescale corresponds to the last few percent to tens of percent of mass accretion for a Milky WayYtype galaxy). Specifically, the frequency, sky coverage, and fraction of stars in substructures in the stellar halo as a function of surface brightness are indicators of the importance of recent merging and of the luminosity function of infalling dwarfs. The morphology of features serves as a guide to the orbital distribution of those dwarfs. Constraints on the earlier merger history (>8 Gyr ago) can be gleaned from the abundance patterns in halo stars: within our models, dramatic differences in the dominant epoch of accretion or luminosity function of progenitor objects leave clear signatures in the [/Fe] and [Fe/H] distributions of the stellar halo; halos dominated by very early accretion have higher average [/Fe], while those dominated by high-luminosity satellites have higher [Fe/H]. This insight can be applied to reconstruct much about the merger histories of nearby galaxies from current and future data sets.

Current models of galaxy formation predict satellite galaxies in groups and clusters that are redder than observed. We investigate the effect on the colours of satellite galaxies produced by the ram-pressure stripping of their hot-gaseous atmospheres as the satellites orbit within their parent halo. We incorporate a model of the stripping process based on detailed hydrodynamic simulations within the Durham semi-analytic model of galaxy formation. The simulations show that the environment in groups and clusters is less aggressive than previously assumed. The main uncertainty in the model is the treatment of gas expelled by supernovae. With reasonable assumptions for the stripping of this material, we find that satellite galaxies are able to retain a significant fraction of their hot gas for several Gyr, thereby replenishing their reservoirs of cold, star-forming gas and remaining blue for a relatively long period of time. A bimodal distribution of galaxy colours, similar to that observed in Sloan Digital Sky Survey data, is established and the colours of the satellite galaxies are in good agreement with the data. In addition, our model naturally accounts for the observed dependence of satellite colours on environment, from small groups to high-mass clusters.

We use the Galaxies‐Intergalactic Medium Interaction Calculation (gimic) suite of cosmological hydrodynamical simulations to study the formation of stellar spheroids of Milky Way mass disc galaxies. The simulations contain accurate treatments of metal‐dependent radiative cooling, star formation, supernova feedback and chemodynamics, and the large volumes that have been simulated yield an unprecedentedly large sample of ≈400 simulated ∼L* disc galaxies. The simulated galaxies are surrounded by low‐mass, low surface brightness stellar haloes that extend out to ∼100 kpc and beyond. The diffuse stellar distributions bear a remarkable resemblance to those observed around the Milky Way, M31 and other nearby galaxies, in terms of mass density, surface brightness and metallicity profiles. We show that in situ star formation typically dominates the stellar spheroids by mass at radii of r≲ 30 kpc, whereas accretion of stars dominates at larger radii and this change in origin induces a change in the slope of the surface brightness and metallicity profiles, which is also present in the observational data. The system‐to‐system scatter in the in situ mass fractions of the spheroid, however, is large and spans over a factor of 4. Consequently, there is a large degree of scatter in the shape and normalization of the spheroid density profile within r≲ 30 kpc (e.g. when fitted by a spherical power‐law profile, the indices range from −2.6 to −3.4). We show that the in situ mass fraction of the spheroid is linked to the formation epoch of the system. Dynamically, older systems have, on average, larger contributions from in situ star formation, although there is significant system‐to‐system scatter in this relationship. Thus, in situ star formation likely represents the solution to the long‐standing failure of pure accretion‐based models to reproduce the observed properties of the inner spheroid.

We use the galaxy stellar mass and halo merger tree information from the semi-analytic model galaxy catalogue of Font et al. (2009) to examine the accretion of galaxies into a large sample of groups and clusters, covering a wide range in halo mass (10E12.9 to 10E15.3 Msun/h), and selected from each of four redshift epochs (z=0, 0.5, 1.0 and 1.5). We find that clusters at all examined redshifts have accreted a significant fraction of their final galaxy populations through galaxy groups. A 10E14.5 Msun/h mass cluster at z=0 has, on average, accreted ~ 40% of its galaxies (Mstellar > 10E9 Msun/h) from halos with masses greater than 10E13 Msun/h. Further, the galaxies which are accreted through groups are more massive, on average, than galaxies accreted through smaller halos or from the field population. We find that at a given epoch, the fraction of galaxies accreted from isolated environments is independent of the final cluster or group mass. In contrast, we find that observing a cluster of the same halo mass at each redshift epoch implies different accretion rates of isolated galaxies, from 5-6 % per Gyr at z=0 to 15% per Gyr at z=1.5. We find that combining the existence of a Butcher Oemler effect at z=0.5 and the observations that galaxies within groups display significant environmental effects with galaxy accretion histories justifies striking conclusions. Namely, that the dominant environmental process must begin to occur in halos of 10E12 -- 10E13 Msun/h, and act over timescales of > 2 Gyrs. This argues in favor of a mechanism like "strangulation", in which the hot halo of a galaxy is stripped upon infalling into a more massive halo . This simple model predicts that by z=1.5 galaxy groups and clusters will display little to no environmental effects.Comment: Accepted for publication in MNRAS. 15 pages, 13 figure