We present six simulations of galactic stellar haloes formed by the tidal disruption of accreted dwarf galaxies in a fully cosmological setting. Our model is based on the Aquarius project, a suite of high resolution N-body simulations of individual dark matter haloes. We tag subsets of particles in these simulations with stellar populations predicted by the Galform semi-analytic model. Our method self-consistently tracks the dynamical evolution and disruption of satellites from high redshift. The luminosity function and structural properties of surviving satellites, which agree well with observations, suggest that this technique is appropriate. We find that accreted stellar haloes are assembled between 1

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We have combined the semi-analytic galaxy formation model of Guo et al. (2011) with the particle-tagging technique of Cooper et al. (2010) to predict galaxy surface brightness profiles in a representative sample of ∼ 1900 massive dark matter haloes (10 12 -10 14 M ) from the Millennium II ΛCDM N-body simulation. Here we present our method and basic results focusing on the outer regions of galaxies, consisting of stars accreted in mergers. These simulations cover scales from the stellar haloes of Milky Way-like galaxies to the 'cD envelopes' of groups and clusters, and resolve low surface brightness substructure such as tidal streams. We find that the surface density of accreted stellar mass around the central galaxies of dark matter haloes is well described by a Sèrsic profile, the radial scale and amplitude of which vary systematically with halo mass (M 200 ). The total stellar mass surface density profile breaks at the radius where accreted stars start to dominate over stars formed in the galaxy itself. This break disappears with increasing M 200 because accreted stars contribute more of the total mass of galaxies, and is less distinct when the same galaxies are averaged in bins of stellar mass, because of scatter in the relation between M and M 200 . To test our model we have derived average stellar mass surface density profiles for massive galaxies at z ≈ 0.08 by stacking SDSS images. Our model agrees well with these stacked profiles and with other data from the literature and makes predictions that can be more rigorously tested by future surveys that extend the analysis of the outer structure of galaxies to fainter isophotes. We conclude that it is likely that the outer structure of the spheroidal components of galaxies is largely determined by collisionless merging during their hierarchical assembly.

We present the second data release from the GALEX Arecibo SDSS Survey (GASS), an ongoing large Arecibo program to measure the Hi properties for an unbiased sample of ∼1000 galaxies with stellar masses greater than 10 10 M and redshifts 0.025 < z < 0.05. GASS targets are selected from the Sloan Digital Sky Survey (SDSS) spectroscopic and Galaxy Evolution Explorer (GALEX) imaging surveys, and are observed until detected or until a gas mass fraction limit of a few per cent is reached. This second data installment includes new Arecibo observations of 240 galaxies, and marks the 50% of the complete survey. We present catalogs of the Hi, optical and ultraviolet parameters for these galaxies, and their Hi-line profiles. Having more than doubled the size of the sample since the first data release, we also revisit the main scaling relations of the Hi mass fraction with galaxy stellar mass, stellar mass surface density, concentration index, and NUV−r color, as well as the gas fraction plane introduced in our earlier work.

We present the final data release from the GALEX Arecibo SDSS Survey (GASS), a large Arecibo program that measured the Hi properties for an unbiased sample of ∼800 galaxies with stellar masses greater than 10 10 M ⊙ and redshifts 0.025 < z < 0.05. This release includes new Arecibo observations for 250 galaxies. We use the full GASS sample to investigate environmental effects on the cold gas content of massive galaxies at fixed stellar mass. The environment is characterized in terms of dark matter halo mass, obtained by cross-matching our sample with the SDSS group catalog of Yang et al. Our analysis provides, for the first time, clear statistical evidence that massive galaxies located in halos with masses of 10 13 −10 14 M ⊙ have at least 0.4 dex less Hi than objects in lower density environments. The process responsible for the suppression of gas in group galaxies most likely drives the observed quenching of the star formation in these systems. Our findings strongly support the importance of the group environment for galaxy evolution, and have profound implications for semi-analytic models of galaxy formation, which currently do not allow for stripping of the cold interstellar medium in galaxy groups.

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We present a model for the satellites of the Milky Way in which galaxy formation is followed using semi-analytic techniques applied to the six high-resolution N-body simulations of galactic halos of the Aquarius project. The model, calculated using the GALFORM code, incorporates improved treatments of the relevant physics in the ΛCDM cosmogony, particularly a self-consistent calculation of reionization by UV photons emitted by the forming galaxy population, including the progenitors of the central galaxy. Along the merger tree of each halo, the model calculates gas cooling (by Compton scattering off cosmic microwave background photons, molecular hydrogen and atomic processes), gas heating (from hydrogen photoionization and supernova energy), star formation and evolution. The evolution of the intergalactic medium is followed simultaneously with that of the galaxies. Star formation in the more massive progenitor subhalos is suppressed primarily by supernova feedback, while for smaller subhalos it is suppressed primarily by photoionization due to external and internal sources. The model is constrained to match a wide range of properties of the present day galaxy population as a whole, but at high redshift it requires an escape fraction of UV photons near unity in order completely to reionize the universe by redshift z ∼ > 8. In the most successful model the local sources photoionize the pre-galactic region completely by z ≃ 10. In addition to the luminosity function of Milky Way satellites, the model matches their observed luminosity-metallicity relation, their radial distribution and the inferred values of the mass within 300 pc, which in the models increase slowly but significantly with luminosity. There is a large variation in satellite properties from halo to halo, with the luminosity function, for example, varying by a factor of ∼ 2 among the six simulations.

We simulate the phase-space distribution of stellar mass in nine massive ΛCDM galaxy clusters by applying the semi-analytic particle tagging method of Cooper et al. to the Phoenix suite of high-resolution N -body simulations (M 200 ≈ 7.5-33 × 10 14 M ). The resulting surface brightness (SB) profiles of brightest cluster galaxies (BCGs) match well to observations. On average, stars formed in galaxies accreted by the BCG account for 90 per cent of its total mass (the remainder is formed in situ). In circular BCG-centred apertures, the superposition of multiple debris clouds (each 10 per cent of the total BCG mass) from different progenitors can result in an extensive outer diffuse component, qualitatively similar to a 'cD envelope'. These clouds typically originate from tidal stripping at z 1 and comprise both streams and the extended envelopes of other massive galaxies in the cluster. Stars at very low SB contribute a significant fraction of the total cluster stellar mass budget: in the central 1 Mpc 2 of a z ∼ 0.15 cluster imaged at SDSS-like resolution, our fiducial model predicts 80-95 per cent of stellar mass below a SB of µ V ∼ 26.5 mag arcsec −2 is associated with accreted stars in the envelope of the BCG. The ratio of BCG stellar mass (including this diffuse component) to total cluster stellar mass is ∼ 30 per cent.