Aims. The goal of this work is to measure the evolution of the Galaxy Stellar Mass Function and of the resulting Stellar Mass Density up to redshift 4, in order to study the assembly of massive galaxies in the high redshift Universe. Methods. We have used the GOODS-MUSIC catalog, containing ∼3000 Ks-selected galaxies with multi-wavelength coverage extending from the U band to the Spitzer 8 µm band, of which 27% have spectroscopic redshifts and the remaining fraction have accurate photometric redshifts. On this sample we have applied a standard fitting procedure to measure stellar masses. We compute the Galaxy Stellar Mass Function and the resulting Stellar Mass Density up to redshift 4, taking into proper account the biases and incompleteness effects. Results. Within the well known trend of global decline of the Stellar Mass Density with redshift, we show that the decline of the more massive galaxies may be described by an exponential timescale of 6 Gyr up to z 1.5, and proceeds much faster thereafter, with an exponential timescale of 0.6 Gyr. We also show that there is some evidence for a differential evolution of the Galaxy Stellar Mass Function, with low mass galaxies evolving faster than more massive ones up to z 1−1.5 and that the Galaxy Stellar Mass Function remains remarkably flat (i.e. with a slope close to the local one) up to z 1−1.3. Conclusions. The observed behaviour of the Galaxy Stellar Mass Function is consistent with a scenario where about 50% of presentday massive galaxies formed at a vigorous rate in the epoch between redshift 4 and 1.5, followed by a milder evolution until the present-day epoch.

It has been widely claimed that several lines of observational evidence point towards a ‘downsizing’ of the process of galaxy formation over cosmic time. This behaviour is sometimes termed ‘antihierarchical’, and contrasted with the ‘bottom‐up’ (small objects form first) assembly of the dark matter structures in cold dark matter (CDM) models. In this paper, we address three different kinds of observational evidence that have been described as ‘downsizing’: the stellar mass assembly (i.e. more massive galaxies assemble at higher redshift with respect to low‐mass ones), star formation rate (SFR) (i.e. the decline of the specific star formation rate is faster for more massive systems) and the ages of the stellar populations in local galaxies (i.e. more massive galaxies host older stellar populations). We compare a broad compilation of available data sets with the predictions of three different semi‐analytic models of galaxy formation within the ΛCDM framework. In the data, we see only weak evidence at best of ‘downsizing’ in stellar mass and in SFR. Despite the different implementations of the physical recipes, the three models agree remarkably well in their predictions. We find that, when observational errors on stellar mass and SFR are taken into account, the models acceptably reproduce the evolution of massive galaxies (M > 1011 M⊙ in stellar mass), over the entire redshift range that we consider (0 ≲z≲ 4). However, lower mass galaxies, in the stellar mass range 109–1011 M⊙, are formed too early in the models and are too passive at late times. Thus, the models do not correctly reproduce the downsizing trend in stellar mass or the archaeological downsizing, while they qualitatively reproduce the mass‐dependent evolution of the SFR. We demonstrate that these discrepancies are not solely due to a poor treatment of satellite galaxies but are mainly connected to the excessively efficient formation of central galaxies in high‐redshift haloes with circular velocities ∼100–200 km s−1. We conclude that some physical processes operating on these mass scales – most probably star formation and/or supernova feedback – are not yet properly treated in these models.

Aims. The goal of this work is to infer the star formation properties and the mass assembly process of high redshift (0.3 ≤ z < 2.5) galaxies from their IR emission using the 24 μm band of MIPS-Spitzer. Methods. We used an updated version of the GOODS-MUSIC catalog, which has multiwavelength coverage from 0.3 to 24 μm and either spectroscopic or accurate photometric redshifts. We describe how the catalog has been extended by the addition of mid-IR fluxes derived from the MIPS 24 μm image. We compared two different estimators of the star formation rate (SFR hereafter). One is the total infrared emission derived from 24 μm, estimated using both synthetic and empirical IR templates. The other one is a multiwavelength fit to the full galaxy SED, which automatically accounts for dust reddening and age-star formation activity degeneracies. For both estimates, we computed the SFR density and the specific SFR. Results. We show that the two SFR indicators are roughly consistent, once the uncertainties involved are taken into account. However, they show a systematic trend, IR-based estimates exceeding the fit-based ones as the star formation rate increases. With this new catalog, we show that: a) at z > 0.3, the star formation rate is correlated well with stellar mass, and this relationship seems to steepen with redshift if one relies on IR-based estimates of the SFR; b) the contribution to the global SFRD by massive galaxies increases with redshift up to 2.5, more rapidly than for galaxies of lower mass, but appears to flatten at higher z; c) despite this increase, the most important contributors to the SFRD at any z are galaxies of about, or immediately lower than, the characteristic stellar mass; d) at z 2, massive galaxies are actively star-forming, with a median SFR 300 M yr −1 . During this epoch, our targeted galaxies assemble a substantial part of their final stellar mass; e) the specific SFR (SSFR) shows a clear bimodal distribution. Conclusions. The analysis of the SFR density and the SSFR seems to support the downsizing scenario, according to which high mass galaxies have formed their stars earlier and more rapidly than their low mass counterparts. A comparison with renditions of theoretical simulations of galaxy formation and evolution indicates that these models follow the global increase in the SSFR with redshift and predict the existence of quiescent galaxies even at z > 1.5. However, the average SSFR is systematically underpredicted by all models considered.

We present the Model for the Rise of Galaxies and Active Nuclei (MORGANA), a new code for the formation and evolution of galaxies and active galactic nuclei (AGNs). Starting from the merger trees of dark matter (DM) haloes and a model for the evolution of substructure within the haloes, the complex physics of baryons is modelled with a set of state-of-the-art models that describe the mass, metal and energy flows between the various components (baryonic halo, bulge, disc) and phases (cold and hot gas, stars) of a galaxy. These flows are then numerically integrated to produce predictions for the evolution of galaxies. The processes of shock-heating and cooling, star formation, feedback, galactic winds and superwinds, accretion on to black holes and AGN feedback are described by new models. In particular, the evolution of the halo gas explicitly follows the thermal and kinetic energies of the hot and cold phases, while star formation and feedback follow the results of the multiphase model recently proposed by Monaco. The increased level of sophistication of these models allows to move from a phenomenological description of gas physics, based on simple scalings with the depth of the DM halo potential, towards a fully physically motivated one. We deem that this is fully justified by the level of maturity and rough convergence reached by the latest versions of numerical and semi-analytic models of galaxy formation. The comparison of the predictions of MORGANA with a basic set of galactic data reveals from the one hand an overall rough agreement, and from the other hand highlights a number of well-or less-known problems: (i) producing the cut-off of the luminosity function requires to force the quenching of the late cooling flows by AGN feedback, (ii) the normalization of the Tully-Fisher relation of local spirals cannot be recovered unless the DM haloes are assumed to have a very low concentration, (iii) the mass function of H I gas is not easily fitted at small masses, unless a similarly low concentration is assumed, (iv) there is an excess of small elliptical galaxies at z = 0. These discrepancies, more than the points of agreement with data, give important clues on the missing ingredients of galaxy formation.

It is generally assumed that the central galaxy in a dark matter halo, that is the galaxy with the lowest specific potential energy, is also the brightest halo galaxy (BHG), and that it resides at rest at the centre of the dark matter potential well. This central galaxy paradigm (CGP) is an essential assumption made in various fields of astronomical research. In this paper, we test the validity of the CGP using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey. For each group, we compute two statistics,  and , which quantify the offsets of the line‐of‐sight velocities and projected positions of brightest group galaxies relative to the other group members. By comparing the cumulative distributions of  and  to those obtained from detailed mock group catalogues, we rule out the null hypothesis that the CGP is correct. Rather, the data indicate that in a non‐zero fraction fBNC(M) of all haloes of mass M the BHG is not the central galaxy, but instead a satellite galaxy. In particular, we find that fBNC increases from ∼0.25 in low‐mass haloes (1012 h−1≤M≲ 2 × 1013 h−1 M⊙) to ∼0.4 in massive haloes (M≳ 5 × 1013 h−1 M⊙). We show that these values of fBNC are uncomfortably high compared to predictions from halo occupation statistics and from semi‐analytical models of galaxy formation. We end by discussing various implications of a non‐zero fBNC(M), with an emphasis on the halo masses inferred from satellite kinematics.

We investigate the correlation of star formation quenching with internal galaxy properties and large‐scale environment (halo mass) in empirical data and theoretical models. We make use of the halo‐based group catalogue of Yang and collaborators, which is based on the Sloan Digital Sky Survey. Data from the Galaxy evolution explorer are also used to extract the recent star formation rate. In order to investigate the environmental effects, we examine the properties of ‘central’ and ‘satellite’ galaxies separately. For central galaxies, we are unable to conclude whether star formation quenching is primarily connected with halo mass or stellar mass, because these two quantities are themselves strongly correlated. For satellite galaxies, a nearly equally strong dependence on halo mass and stellar mass is seen. We make the same comparison for five different semi‐analytic models based on three independently developed codes. We find that the models with active galactic nuclei feedback reproduce reasonably well the dependence of the fraction of central red and passive galaxies on halo mass and stellar mass. However, for satellite galaxies, the same models badly overproduce the fraction of red/passive galaxies and do not reproduce the empirical trends with stellar mass or halo mass. This satellite overquenching problem is caused by the too‐rapid stripping of the satellites' hot gas haloes, which leads to rapid strangulation of star formation.

One major problem of current theoretical models of galaxy formation is given by their inability to reproduce the apparently 'anti-hierarchical' evolution of galaxy assembly: massive galaxies appear to be in place since z ∼ 3, while a significant increase of the number densities of low mass galaxies is measured with decreasing redshift. In this work, we perform a systematic analysis of the influence of different stellar feedback schemes, carried out in the framework of Gaea, a new semi-analytic model of galaxy formation. It includes a self-consistent treatment for the timings of gas, metal and energy recycling, and for the chemical yields. We show this to be crucial to use observational measurements of the metallicity as independent and powerful constraints for the adopted feedback schemes. The observed trends can be reproduced in the framework of either a strong ejective or preventive feedback model. In the former case, the gas ejection rate must decrease significantly with cosmic time (as suggested by parametrizations of the cosmological "FIRE" simulations). Irrespective of the feedback scheme used, our successful models always imply that up to 60-70 per cent of the baryons reside in an 'ejected' reservoir and are unavailable for cooling at high redshift. The same schemes predict physical properties of model galaxies (e.g. gas content, colour, age, and metallicity) that are in much better agreement with observational data than our fiducial model. The overall fraction of passive galaxies is found to be primarily determined by internal physical processes, with environment playing a secondary role.

We used Early Release Science (ERS) observations taken with the Wide Field Camera 3 (WFC3) in the GOODS-S field to study the galaxy stellar mass function (GSMF) at 0.6 ≤ z < 4.5. Deep WFC3 near-IR data (for Y as faint as 27.3, J and H as faint as 27.4 AB mag at 5σ), as well as deep K S (as faint as 25.5 at 5σ) Hawk-I band data, provide an exquisite data set with which determine in an unprecedented way the low-mass end of the GSMF, allowing an accurate probe of masses as low as M * 7.6 × 10 9 M at z ∼ 3. Although the area used is relatively small (∼33 arcmin 2 ), we found generally good agreement with previous studies on the entire mass range. Our results show that the slope of the faint-end increases with redshift, from α = −1.44 ± 0.03 at z ∼ 0.8 to α = −1.86 ± 0.16 at z ∼ 3, although indications exist that it does not steepen further between z ∼ 3 and z ∼ 4. This result is insensitive to any uncertainty in the M * parameter. The steepness of the GSMF faint-end solves the well-known disagreement between the stellar mass density (SMD) and the integrated star-formation history at z > 2. However, we confirm that there appears to be an excess of integrated star formation with respect to the SMD at z < 2, by a factor of ∼2−3. Our comparison of the observations with theoretical predictions shows that the models forecast a greater abundance of low mass galaxies, at least up to z ∼ 3, as well as a dearth of massive galaxies at z ∼ 4 with respect to the data, and that the predicted SMD is generally overestimated at z < ∼ 2.