We study the growth of galaxy masses, via gas accretion and galaxy mergers. We introduce a toy model that describes (in a single equation) how much baryonic mass is accreted and retained into galaxies as a function of halo mass and redshift. In our model, the evolution of the baryons differs from that of the dark matter because 1) gravitational shock heating and AGN jets suppress gas accretion mainly above a critical halo mass of M shock ∼ 10 12 M ; 2) the intergalactic medium after reionisation is too hot for accretion onto haloes with circular velocities v circ < ∼ 40 km s −1 ; 3) stellar feedback drives gas out of haloes, mainly those with v circ < ∼ 120 km s −1 . We run our model on the merger trees of the haloes and sub-haloes of a high-resolution dark matter cosmological simulation. The galaxy mass is taken as the maximum between the mass given by the toy model and the sum of the masses of its progenitors (reduced by tidal stripping). Designed to reproduce the present-day stellar mass function of galaxies, our model matches fairly well the evolution of the cosmic stellar density. It leads to the same z = 0 relation between central galaxy stellar and halo mass as the one found by abundance matching and also as that previously measured at high mass on SDSS centrals. Our model also predicts a bimodal distribution (centrals and satellites) of stellar masses for given halo mass, in very good agreement with SDSS observations. The relative importance of mergers depends strongly on stellar mass (more than on halo mass). Massive galaxies with m stars > m crit ∼ Ω b /Ω m M shock ∼ 10 11 M acquire most of their final mass through mergers (mostly major and gas-poor), as expected from our model's shutdown of gas accretion at high halo masses. However, although our mass resolution should see the effects of mergers down to m stars 10 10.6 h −1 M , we find that mergers are rare for m stars 10 11 h −1 M . This is a consequence of the curvature of the stellar vs. halo mass relation set by the physical processes of our toy model and found with abundance matching. So gas accretion must be the dominant growth mechanism for intermediate and low mass galaxies, including dwarf ellipticals in clusters. The contribution of galaxy mergers terminating in haloes with mass M halo < M shock (thus presumably gas-rich) to the mass buildup of galaxies is small at all masses, but accounts for the bulk of the growth of ellipticals of intermediate mass (∼10 10.5 h −1 M ), which we predict must be the typical mass of ULIRGs.

We present a detailed analysis of the properties of tidally stripped material from disrupting substructure haloes or subhaloes in a sample of high-resolution cosmological N-body host haloes ranging from galaxy-to cluster-mass scales. We focus on devising methods to recover the infall mass and infall eccentricity of subhaloes from the properties of their tidally stripped material (i.e. tidal streams). Our analysis reveals that there is a relation between the scatter of stream particles about the best-fitting debris plane and the infall mass of the progenitor subhalo. This allows us to reconstruct the infall mass from the spread of its tidal debris in space. We also find that the spread in radial velocities of the debris material (as measured by an observer located at the centre of the host) correlates with the infall eccentricity of the subhalo, which allows us to reconstruct its orbital parameters. We devise an automated method to identify the leading and trailing arms that can, in principle at least, be applied to observations of stellar streams from satellite galaxies. This method is based on the energy distribution of material in the tidal stream. Using this method, we show that the masses associated with the leading and trailing arms differ. While our analysis indicates that tidal streams can be used to recover certain properties of their progenitor subhaloes (and consequently satellites), we do not find strong correlations between host halo properties and stream properties. This likely reflects the complicated relationship between the stream and the host, which in a cosmological context is characterized by a complex mass accretion history, an asymmetric mass distribution and the abundance of substructure. Finally, we confirm that the so-called 'backsplash' subhalo population is present not only in galaxy cluster haloes but also in galaxy haloes. The orbits of backsplash subhaloes brought them inside the virial radius of their host at some earlier time, but they now reside in its outskirts at the present-day, beyond the virial radius. Both backsplash and bound subhaloes experience similar mass loss, but the contribution of the backsplash subhaloes to the overall tidal debris field is negligible.

We present a detailed analysis of the velocity distribution and orientation of orbits of subhaloes in high resolution cosmological simulations of dark matter haloes. We find a trend for substructure to preferentially revolve in the same direction as the sense of rotation of the host halo: there is an excess of prograde satellite haloes. Throughout our suite of nine host haloes (eight cluster sized objects and one galactic halo) there are on average 59% of the satellites corotating with the host. Even when including satellites out to five virial radii of the host, the signal still remains pointing out the relation of the signal with the infall pattern of subhaloes. However, the fraction of prograde satellites weakens to about 53% when observing the data along a (random) line-of-sight and deriving the distributions in a way an observer would infer them. This decrease in the observed prograde fraction has its origin in the technique used by the observer to determine the sense of rotation, which results in a possible misclassification of non-circular orbits. We conclude that the existence of satellites on corotating orbits is another prediction of the cold dark matter structure formation scenario, although there will be difficulties to verify it observationally. Since the galactic halo simulation gave the same result as the cluster-sized simulations, we assume that the fraction of prograde orbits is independent of the scale of the system, though more galactic simulations would be necessary to confirm this.Comment: 16 pages, 9 figures, accepted by MNRAS; extended comparison with previous work (mistake corrected) and observations, typos correcte