We present a detailed analysis of the influence of the environment and of the environmental history on quenching star formation in central and satellite galaxies in the local Universe. We take advantage of publicly available galaxy catalogues obtaine
d from applying a galaxy formation model to the Millennium simulation. In addition to halo mass, we consider the local density of galaxies within various fixed scales. Comparing our model predictions to observational data (SDSS), we demonstrate that the models are failing to reproduce the observed density dependence of the quiescent galaxy fraction in several aspects: for most of the stellar mass ranges and densities explored, models cannot reproduce the observed similar behaviour of centrals and satellites, they slightly under-estimate the quiescent fraction of centrals and significantly over-estimate that of satellites. We show that in the models, the density dependence of the quiescent central galaxies is caused by a fraction of backsplash centrals which have been satellites in the past (and were thus suffering from environmental processes). Turning to satellite galaxies, the density dependence of their quiescent fractions reflects a dependence on the time spent orbiting within a parent halo of a particular mass, correlating strongly with halo mass and distance from the halo centre. Comparisons with observational estimates suggest relatively long gas consumption time scales of roughly 5 Gyr in low mass satellite galaxies. The quenching time scales decrease with increasing satellite stellar mass. Overall, a change in modelling both internal processes (star formation and feedback) and environmental processes (e.g. making them dependent on dynamical friction time-scales and preventing the re-accretion of gas onto backsplash galaxies) is required for improving currently used galaxy formation models.
We study the evolution of disc galaxies in group environments under the effect of both the global tidal field and close-encounters between galaxies, using controlled N-body simulations of isolated mergers. We find that close-range encounters between
galaxies are less frequent and less damaging to disc galaxies than originally expected, since they mostly occur when group members have lost a significant fraction of their initial mass to tidal stripping. We also find that group members mostly affect disc galaxies indirectly by modifying their common global tidal field. Different initial orbital parameters of group members introduce a significant scatter in the evolution of general properties of disc galaxies around a median evolution that is similar to when only the effect of the global tidal field is included. Close-encounters introduce a high variability in the properties of disc galaxies, even slowing their evolution in some cases, and could wash out correlations between galaxy properties and the group total mass. The combined effect of the global tidal field and close-encounters appears to be inefficient at forming/enhancing central stellar bulges. This implies that bulges of S0 galaxies should be mostly composed by young stars, which is consistent with recent observations.
We compare three analytical prescriptions for merger times available from the literature to simulations of isolated mergers. We probe three different redshifts, and several halo concentrations, mass ratios, orbital circularities and orbital energies
of the satellite. We find that prescriptions available in the literature significantly under-predict long timescales for mergers at high redshift. We argue that these results have not been highlighted previously either because the evolution of halo concentration of satellite galaxies has been neglected (in previous isolated merger simulations), or because long merger times and mergers with high initial orbital circularities are under-represented (for prescriptions based on cosmological simulations). Motivated by the evolution of halo concentration at fixed mass, an explicit dependence on redshift added as t_merger,modified(z) = (1+z)^0.44 t_merger to the prescription based on isolated mergers gives a significant improvement in the predicted merger times up to ~20 t_dyn in the redshift range 0<z<2. When this modified prescription is used to compute galaxy stellar mass functions, we find that it leads up to a 25 per cent increase in the number of low mass galaxies surviving at z=0, and a 10 per cent increase for more massive galaxies. This worsen the known over-prediction in the number of low mass galaxies by hierarchical models of galaxy formation.
In this study, we have carried out a detailed, statistical analysis of isolated model galaxies, taking advantage of publicly available hierarchical galaxy formation models. To select isolated galaxies, we employ 2D methods widely used in the observat
ional literature, as well as a more stringent 3D isolation criterion that uses the full 3D-real space information. In qualitative agreement with observational results, isolated model galaxies have larger fractions of late-type, star forming galaxies with respect to randomly selected samples of galaxies with the same mass distribution. We also find that the samples of isolated model galaxies typically contain a fraction of less than 15 per cent of satellite galaxies, that reside at the outskirts of their parent haloes where the galaxy number density is low. Projection effects cause a contamination of 2D samples of about 18 per cent, while we estimate a typical completeness of 65 per cent. Our model isolated samples also include a very small (few per cent) fraction of bulge dominated galaxies (B/T > 0.8) whose bulges have been built mainly by minor mergers. Our study demonstrates that about 65-70 per cent of 2D isolated galaxies that are classified as isolated at z = 0 have indeed been completely isolated since z = 1 and only 7 per cent have had more than 3 neighbours within a comoving radius of 1 Mpc. Irrespectively of the isolation criteria, roughly 45 per cent of isolated galaxies have experienced at least one merger event in the past (most of the mergers are minor, with mass ratios between 1:4 and 1:10). The latter point validates the approximation that isolated galaxies have been mainly influenced by internal processes.
We combine the six high-resolution Aquarius dark matter simulations with a semi-analytic galaxy formation model to investigate the properties of the satellites of Milky Way-like galaxies. We find good correspondence with the observed luminosity funct
ion, luminosity-metallicity relation and radial distribution of the Milky Way satellites. The star formation histories of the dwarf galaxies in our model vary widely, in accordance with what is seen observationally. Ram-pressure stripping of hot gas from the satellites leaves a clear imprint of the environment on the characteristics of a dwarf galaxy. We find that the fraction of satellites dominated by old populations of stars matches observations well. However, the internal metallicity distributions of the model satellites appear to be narrower than observed. This may indicate limitations in our treatment of chemical enrichment, which is based on the instantaneous recycling approximation. Our model works best if the dark matter halo of the Milky Way has a mass of ~8 x 10^11 Msun, in agreement with the lower estimates from observations. The galaxy that resembles the Milky Way the most also has the best matching satellite luminosity function, although it does not contain an object as bright as the SMC or LMC. Compared to other semi-analytic models and abundance matching relations we find that central galaxies reside in less massive haloes, but the halo mass-stellar mass relation in our model is consistent both with hydrodynamical simulations and with recent observations.
We present results from high--resolution cosmological hydrodynamical simulations of a Milky--Way-sized halo, aimed at studying the effect of feedback on the nature of gas accretion. Simulations include a model of inter-stellar medium and star formati
on, in which SN explosions provide effective thermal feedback. We distinguish between gas accretion onto the halo, which occurs when gas particles cross the halo virial radius, and gas accretion onto the central galaxy, which takes place when gas particles cross the inner one-tenth of the virial radius. Gas particles can be accreted through three different channels, depending on the maximum temperature value, $T_{rm max}$, reached during the particles past evolution: a cold channel for $T_{rm max}<2.5 times 10^5$ K, a hot one for $T>10^6$K, and a warm one for intermediate values of $T_{rm max}$. We find that the warm channel is at least as important as the cold one for gas accretion onto the central galaxy. This result is at variance with previous findings that the cold mode dominates gas accretion at high redshift. We ascribe this difference to the different supernova feedback scheme implemented in our simulations. While results presented so far in the literature are based on uneffective SN thermal feedback schemes and/or the presence of a kinetic feedback, our simulations include only effective thermal feedback. We argue that observational detections of a warm accretion mode in the high--redshift circum-galactic medium would provide useful constraints on the nature of the feedback that regulates star formation in galaxies.
We present the results of a series of numerical simulations aimed to study the evolution of a disc galaxy within the global tidal field of a group environment. Both the disc galaxy and the group are modelled as multi-component, collision-less, N-body
systems, composed by both dark matter and stars. In our simulations, the evolution of disc galaxies is followed as their orbits sink towards the group centre, under the effect of dynamical friction. We explore a broad parameter space, covering several aspects of the galaxy-group interaction that are potentially relevant to galaxy evolution. Namely, prograde and retrograde orbits, orbital eccentricities, disc inclination, role of a central bulge in discs, internal disc kinematics, and galaxy-to-group mass ratios. We find that significant disc transformations occur only after the mean density of the group, measured within the orbit of the galaxy, exceeds ~0.3-1 times the central mean density of the galaxy. The morphological evolution of discs is found to be strongly dependent on the initial inclination of the disc with respect to its orbital plane. That is, discs on face-on and retrograde orbits are shown to retain longer their disc structures and kinematics, in comparison to prograde discs. This suggests that after interacting with the global tidal field alone, a significant fraction of disc galaxies should be found in the central regions of groups. Prominent central bulges are not produced, and pre-existing bulges are not enhanced in discs after the interaction with the group. Assuming that most S0 are formed in group environments, this implies that prominent bulges should be formed mostly by young stars, created only after a galaxy has been accreted by a group. Finally, contrary to some current implementations of tidal stripping in semi-analytical models of galaxy evolution, we find that more massive galaxies suffer more tidal stripping.
We study the colour-magnitude relation (CMR) for a sample of 172 morphologically-classified E/S0 cluster galaxies from the ESO Distant Cluster Survey (EDisCS) at 0.4<z<0.8. The intrinsic colour scatter about the CMR is very small (0.076) in rest-fram
e U-V. Only 7% of the galaxies are significantly bluer than the CMR. The scarcity of blue S0s indicates that, if they are the descendants of spirals, these were already red when they became S0s. We observe no dependence of the CMR scatter with redshift or cluster velocity dispersion. This implies that by the time cluster E/S0s achieve their morphology, the vast majority have already joined the red sequence. We estimate the galaxy formation redshift z_F for each cluster and find that it does not depend on the cluster velocity dispersion. However, z_F increases weakly with cluster redshift. This trend becomes clearer when including higher-z clusters from the literature, suggesting that, at any given z, in order to have a population of fully-formed E and S0s they needed to have formed most of their stars 2-4 Gyr prior to observation. In other words, the galaxies that already have early-type (ET) morphologies also have reasonably-old stellar populations. This is partly a manifestation of the progenitor bias, but also a consequence of the fact that the vast majority of the ETs in clusters (in particular the massive ones) were already red by the time they achieved their morphology. E and S0 galaxies exhibit very similar colour scatter, implying similar stellar population ages. We also find that fainter ETs finished forming their stars later, consistent with the cluster red sequence being built over time and the brightest galaxies reaching the red sequence earlier than fainter ones. Finally, we find that the ET cluster galaxies must have had their star formation truncated over an extended period of at least 1 Gyr. [abridged]
AGN feedback is believed to play an important role in shaping a variety of observed galaxy properties, as well as the evolution of their stellar masses and star formation rates. In particular, in the current theoretical paradigm of galaxy formation,
AGN feedback is believed to play a crucial role in regulating the levels of activity in galaxies, in relatively massive halos at low redshift. Only in recent years, however, has detailed statistical information on the dependence of galaxy activity on stellar mass, parent halo mass and hierarchy has become available. In this paper, we compare the fractions of galaxies belonging to different activity classes (star-forming, AGN and radio active) with predictions from four different and independently developed semi-analytical models. We adopt empirical relations to convert physical properties into observables (H_alpha emission lines, OIII line strength and radio power). We demonstrate that all models used in this study reproduce the overall distributions of galaxies belonging to different activity classes as a function of stellar mass and halo mass: star forming galaxies and the strongest radio sources are preferentially associated with low-mass and high-mass galaxies/halos respectively. However, model predictions differ from observational measurements in a number of ways. All models used in our study predict that almost every >1.e12 Msun dark matter halo and/or >1.e11 Msun galaxy should host a bright radio source, while only a small fraction of galaxies belong to this class in the data. In addition, radio brightness is expected to depend strongly on the mass of the parent halo mass in the models, while strong and weak radio galaxies are found in similar environments in data. Our results highlight that the distribution of AGN as a function of stellar mass provides one of the most promising discriminants between different gas accretion schemes.
We show that, observationally, the projected local density distribution in high-z clusters is shifted towards higher values compared to clusters at lower redshift. To search for the origin of this evolution, we analyze a sample of haloes selected fro
m the Millennium Simulation and populated using semi-analytic models, investigating the relation between observed projected density and physical 3D density, using densities computed from the 10 and 3 closest neighbours. Both observationally and in the simulations, we study the relation between number of cluster members and cluster mass, and number of members per unit of cluster mass. We find that the observed evolution of projected densities reflects a shift to higher values of the physical 3D density distribution. In turn, this must be related with the globally higher number of galaxies per unit of cluster volume N/V in the past. We show that the evolution of N/V is due to a combination of two effects: a) distant clusters were denser in dark matter (DM) simply because the DM density within R_{200} (~the cluster virial radius) is defined to be a fixed multiple of the critical density of the Universe, and b) the number of galaxies per unit of cluster DM mass is remarkably constant both with redshift and cluster mass if counting galaxies brighter than a passively evolving magnitude limit. Our results highlight that distant clusters were much denser environments than todays clusters, both in galaxy number and mass, and that the density conditions felt by galaxies in virialized systems do not depend on the system mass.
