The distribution of galaxy morphological types is a key test for models of galaxy formation and evolution, providing strong constraints on the relative contribution of different physical processes responsible for the growth of the spheroidal componen
ts. In this paper, we make use of a suite of semi-analytic models to study the efficiency of galaxy mergers in disrupting galaxy discs and building galaxy bulges. In particular, we compare standard prescriptions usually adopted in semi-analytic models, with new prescriptions proposed by Kannan et al., based on results from high-resolution hydrodynamical simulations, and we show that these new implementations reduce the efficiency of bulge formation through mergers. In addition, we compare our model results with a variety of observational measurements of the fraction of spheroid-dominated galaxies as a function of stellar and halo mass, showing that the present uncertainties in the data represent an important limitation to our understanding of spheroid formation. Our results indicate that the main tension between theoretical models and observations does not stem from the survival of purely disc structures (i.e. bulgeless galaxies), rather from the distribution of galaxies of different morphological types, as a function of their stellar mass.
We study the observed cosmic ionizing background as a constraint on the nature of the sources responsible for the reionization of the Universe. In earlier work, we showed that extrapolations of the Ultra-Violet Luminosity Function (LF) of Lyman Break
Galaxies (LBGs) at fixed Lyman continuum photon escape fraction are not able to reproduce the redshift evolution of this background. Here, we employ extrapolations of the high-z LFs to describe the contribution of LBGs to the ionizing photon rate, taking into account the smoothing of the baryonic perturbations, due to the background itself (i.e. the filtering mass), as well as a possible sharp increase of the escape fraction in dwarf galaxies. Under the hypothesis of a dominant contribution of LBGs to cosmic reionization, our results suggest that sources fainter than the current observational limits should be characterised by escape fractions of the order of ~0.1-0.3 (larger than the current estimates for bright galaxies) to account for a z>6 reionization and the measured evolution of cosmic ionizing background, at the same time. The contribution to the background from quasars turns out to be relevant at z<3. Overall, our results support the case for dedicated observations of faint galaxies in the rest-frame UV, in order to better determine their physical properties. Observed escape fractions outside our proposed range bear relevant consequences on the nature of the astrophysical sources responsible for cosmic reionization and/or its buildup process.
Modifications of the equations of general relativity at large distances offer one possibility to explain the observed properties of our Universe without invoking a cosmological constant. Numerous proposals for such modified gravity cosmologies exist,
but often their consequences for structure formation in the non-linear sector are not yet accurately known. In this work, we employ high-resolution numerical simulations of f(R)-gravity models coupled with a semi-analytic model (SAM) for galaxy formation to obtain detailed predictions for the evolution of galaxy properties. The f(R)-gravity models imply the existence of a `fifth-force, which is however locally suppressed, preserving the successes of general relativity on solar system scales. We show that dark matter haloes in f(R)-gravity models are characterized by a modified virial scaling with respect to the LCDM scenario, reflecting a higher dark matter velocity dispersion at a given mass. This effect is taken into account in the SAM by an appropriate modification of the mass--temperature relation. We find that the statistical properties predicted for galaxies (such as the stellar mass function and the cosmic star formation rate) in f(R)-gravity show generally only very small differences relative to LCDM, smaller than the dispersion between the results of different SAM models, which can be viewed as a measure of their systematic uncertainty. We also demonstrate that galaxy bias is not able to disentangle between f(R)-gravity and the standard cosmological scenario. However, f(R)-gravity imprints modifications in the linear growth rate of cosmic structures at large scale, which can be recovered from the statistical properties of large galaxy samples.
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.
[abridged] It has been widely claimed that several lines of observational evidence point towards a downsizing (DS) of the process of galaxy formation over cosmic time. This behavior is sometimes termed anti-hierarchical, and contrasted with the botto
m-up assembly of the dark matter structures in Cold Dark Matter models. In this paper we address three different kinds of observational evidence that have been described as DS: the stellar mass assembly, star formation rate and the ages of the stellar populations in local galaxies. We compare a broad compilation of available data-sets with the predictions of three different semi-analytic models of galaxy formation within the Lambda-CDM framework. In the data, we see only weak evidence at best of DS in stellar mass and in star formation rate. We find that, when observational errors on stellar mass and SFR are taken into account, the models acceptably reproduce the evolution of massive galaxies, over the entire redshift range that we consider. However, lower mass galaxies are formed too early in the models and are too passive at late times. Thus, the models do not correctly reproduce the DS trend in stellar mass or the archaeological DS, 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. [abridged]
In order to gain insight into the physical mechanisms leading to the formation of stars and their assembly in galaxies, we compare the predictions of the MOdel for the Rise of GAlaxies aNd Active nuclei (MORGANA) to the properties of K- and 850 micro
n-selected galaxies (such as number counts, redshift distributions and luminosity functions) by combining MORGANA with the spectrophotometric model GRASIL. We find that it is possible to reproduce the K- and 850 micron-band datasets at the same time and with a standard Salpeter IMF, and ascribe this success to our improved modeling of cooling in DM halos. We then predict that massively star-forming discs are common at z~2 and dominate the star-formation rate, but most of them merge with other galaxies within ~100 Myr. Our preferred model produces an overabundance of bright galaxies at z<1; this overabundance might be connected to the build-up of the diffuse stellar component in galaxy clusters, as suggested by Monaco et al. (2006), but a naive implementation of the mechanism suggested in that paper does not produce a sufficient slow-down of the evolution of these objects. Moreover, our model over-predicts the number of 10^{10}-10^{11} M_sun galaxies at z~1; this is a common behavior of theoretical models as shown by Fontana et al. (2006). These findings show that, while the overall build-up of the stellar mass is correctly reproduced by galaxy formation models, the ``downsizing trend of galaxies is not fully reproduced yet. This hints to some missing feedback mechanism in order to reproduce at the same time the formation of both the massive and the small galaxies.
