We have updated and extended our semi-analytic galaxy formation modelling capabilities and applied them simultaneously to the stored halo/subhalo merger trees of the Millennium and Millennium-II Simulations (MS and MS-II, respectively). These differ by a factor of 125 in mass resolution, allowing explicit testing of resolution effects on predicted galaxy properties. We have revised the treatment of the transition between the rapid infall and cooling flow regimes of gas accretion, of the sizes of bulges, and of gaseous and stellar discs, of supernova feedback, of the transition between central and satellite status as galaxies fall into larger systems, and of gas and star stripping, once they become satellites. Plausible values of efficiency and scaling parameters yield an excellent fit not only to the observed abundance of low-redshift galaxies over five orders of magnitude in stellar mass and 9 mag in luminosity, but also to the observed abundance of Milky Way satellites. This suggests that reionization effects may not be needed to solve the 'missing-satellite' problem, except, perhaps, for the faintest objects. The same model matches the observed large-scale clustering of galaxies as a function of stellar mass and colour. The fit remains excellent down to ∼30 kpc for massive galaxies. For M * < 6 × 10 10 M , however, the model overpredicts clustering at scales below ∼1 Mpc, suggesting that the assumed fluctuation amplitude, σ 8 = 0.9, is too high. The observed difference in clustering between active and passive galaxies is matched quite well for all masses. Galaxy distributions within rich clusters agree between the simulations and match those observed, but only if galaxies without dark matter subhaloes (so-called orphans) are included. Even at MS-II resolution, schemes which assign galaxies only to resolved dark matter subhaloes cannot match observed clusters. Our model predicts a larger passive fraction among low-mass galaxies than is observed, as well as an overabundance of ∼10 10 M galaxies beyond z ∼ 0.6. (The abundance of ∼10 11 M galaxies is matched out to z ∼ 3.) These discrepancies appear to reflect deficiencies in the way star formation rates are modelled.

We use publicly available galaxy merger trees, obtained applying semi‐analytic techniques to a large high‐resolution cosmological simulation, to study the environmental history of group and cluster galaxies. Our results highlight the existence of an intrinsic history bias which makes the nature versus nurture (as well as the mass versus environment) debate inherently ill posed. In particular, we show that (i) surviving massive satellites were accreted later than their less massive counterparts, from more massive haloes and (ii) the mixing of galaxy populations is incomplete during halo assembly, which creates a correlation between the time a galaxy becomes satellite and its present distance from the parent halo centre. The weakest trends are found for the most massive satellites, as a result of efficient dynamical friction and late formation times of massive haloes. A large fraction of the most massive group/cluster members are accreted on to the main progenitor of the final halo as central galaxies, while about half of the galaxies with low and intermediate stellar masses are accreted as satellites. Large fractions of group and cluster galaxies (in particular those of low stellar mass) have therefore been ‘pre‐processed’ as satellites of groups with mass ∼1013 M⊙. To quantify the relevance of hierarchical structure growth on the observed environmental trends, we have considered observational estimates of the passive galaxy fractions and their variation as a function of halo mass and clustercentric distance. Comparisons with our theoretical predictions require relatively long times (∼5–7 Gyr) for the suppression of star formation in group and cluster satellites. It is unclear how such a gentle mode of strangulation can be achieved by simply relaxing the assumption of instantaneous stripping of the hot gas reservoir associated with accreting galaxies, or if the difficulties encountered by recent galaxy formation models in reproducing the observed trends signal a more fundamental problem with the treatment of star formation and feedback in these galaxies.

According to the current paradigm, galaxies initially form as disc galaxies at the centres of their own dark matter haloes. During their subsequent evolution, they may undergo a transformation to a red, early‐type galaxy, thus giving rise to the build‐up of the red sequence. Two important, outstanding questions are (i) which transformation mechanisms are most important and (ii) in what environment do they occur. In this paper, we study the impact of transformation mechanisms that operate only on satellite galaxies, such as strangulation, ram‐pressure stripping and galaxy harassment. Using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey, we compare the colours and concentrations of satellites galaxies to those of central galaxies of the same stellar mass, adopting the hypothesis that the latter are the progenitors of the former. On average, satellite galaxies are redder and more concentrated than central galaxies of the same stellar mass, indicating that satellite‐specific transformation processes do indeed operate. Central‐satellite pairs that are matched in both stellar mass and colour, however, show no average concentration difference, indicating that the transformation mechanisms operating on satellites affect colour more than morphology. We also find that the colour and concentration differences of matched central‐satellite pairs are completely independent of the mass of the host halo (not to be confused with the subhalo) of the satellite galaxy, indicating that satellite‐specific transformation mechanisms are equally efficient in host haloes of all masses. This strongly rules against mechanisms that are thought to operate only in very massive haloes, such as ram‐pressure stripping or harassment. Instead, we argue that strangulation is the main transformation mechanism for satellite galaxies. Finally, we determine the relative importance of satellite quenching for the build‐up of the red sequence. We find that roughly 70 per cent of red‐sequence satellite galaxies with M*∼ 109 h−2 M⊙ had their star formation quenched as satellites. This drops rapidly with increasing stellar mass, reaching virtually zero at M*∼ 1011 h−2 M⊙. Therefore, a very significant fraction of red satellite galaxies were already quenched before they became a satellite.

We use galaxy groups selected from the Sloan Digital Sky Survey to examine the alignment between the orientation of the central galaxy (defined as the brightest group member) and the distribution of satellite galaxies. By construction, we therefore only address the alignment on scales smaller than the halo virial radius. We find a highly significant alignment of satellites with the major axis of their central galaxy. This is in qualitative agreement with the recent study of Brainerd, but inconsistent with several previous studies who detected a preferential minor‐axis alignment. The alignment strength in our sample is strongest between red central galaxies and red satellites. On the contrary, the satellite distribution in systems with a blue central galaxy is consistent with isotropic. We also find that the alignment strength is stronger in more massive haloes and at smaller projected radii from the central galaxy. In addition, there is a weak indication that fainter (relative to the central galaxy) satellites are more strongly aligned. We present a detailed comparison with previous studies, and discuss the implications of our findings for galaxy formation.