Galaxy assembly, stellar feedback and metal enrichment: the view from the GAEA model

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 $z\sim 3$, 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.Comment: 30 pages, 19 figures, accepted for publication by MNRAS; note that corresponding new galaxy catalogues (FIRE model) will soon be made publicly available at http://gavo.mpa-garching.mpg.de/Millennium

Interpreting the possible break in the Black Hole - Bulge mass relation

Recent inspections of local available data suggest that the almost linear relation between the stellar mass of spheroids ($M_{\rm sph}$) and the mass of the super massive Black Holes (BHs) residing at their centres, shows a break below $M_{\rm sph} \sim 10^{10}\ {\rm M}_\odot$, with a steeper, about quadratic relation at smaller masses. We investigate the physical mechanisms responsible for the change in slope of this relation, by comparing data with the results of the semi-analytic model of galaxy formation MORGANA, which already predicted such a break in its original formulation. We find that the change of slope is mostly induced by effective stellar feedback in star-forming bulges. The shape of the relation is instead quite insensitive to other physical mechanisms connected to BH accretion such as disc instabilities, galaxy mergers, Active Galactic Nucleus (AGN) feedback, or even the exact modelling of accretion onto the BH, direct or through a reservoir of low angular momentum gas. Our results support a scenario where most stars form in the disc component of galaxies and are carried to bulges through mergers and disc instabilities, while accretion onto BHs is connected to star formation in the spheroidal component. Therefore, a model of stellar feedback that produces stronger outflows in star-forming bulges than in discs will naturally produce a break in the scaling relation. Our results point to a form of co-evolution especially at lower masses, below the putative break, mainly driven by stellar feedback rather than AGN feedback.Comment: MNRAS accepted, 10 pages, 6 figures, 1 tabl

Nature versus nurture: what regulates star formation in satellite galaxies?

We use our state-of-the-art Galaxy Evolution and Assembly (GAEA) semi-analytic model to study how and on which time-scales star formation is suppressed in satellite galaxies. Our fiducial stellar feedback model, implementing strong stellar driven outflows, reproduces relatively well the variations of passive fractions as a function of galaxy stellar mass and halo mass measured in the local Universe, as well as the `quenching' time-scales inferred from the data. We show that the same level of agreement can be obtained by using an alternative stellar feedback scheme featuring lower ejection rates at high redshift, and modifying the treatment for hot gas stripping. This scheme over-predicts the number densities of low to intermediate mass galaxies. In addition, a good agreement with the observed passive fractions can be obtained only by assuming that cooling can continue on satellites, at the rate predicted considering halo properties at infall, even after their parent dark matter substructure is stripped below the resolution of the simulation. For our fiducial model, the better agreement with the observed passive fractions can be ascribed to: (i) a larger cold gas fraction of satellites at the time of accretion, and (ii) a lower rate of gas reheating by supernovae explosions and stellar winds with respect to previous versions of our model. Our results suggest that the abundance of passive galaxies with stellar mass larger than ~10^10 Msun is primarily determined by the self-regulation between star formation and stellar feedback, with environmental processes playing a more marginal role.Comment: 11 pages, 6 figures, 1 appendix. Accepted for publication in MNRA

On the Evolution of the Star Formation Rate Function of Massive Galaxies. Constraints at 0.4<z<1.8 from the GOODS-MUSIC Catalogue

[abridged] We study the evolution of the Star Formation Rate Function (SFRF) of massive galaxies over the 0.4<z<1.8 redshift range and its implications for our understanding of the physical processes responsible for galaxy evolution. We use multiwavelength observations included in the GOODS-MUSIC catalogue, which provides a suitable coverage of the spectral region from 0.3 to 24 micron and either spectroscopic or photometric redshifts for each object. Individual SFRs have been obtained by combining UV and 24 micron observations, when the latter were available. For all other sources an "SED fitting" SFR estimate has been considered. We then define a stellar mass limited sample, complete in the Mstar>1.e10 Msun range and determine the SFRF using the 1/Vmax algorithm. We define simulated galaxy catalogues based on three different semi-analytical models of galaxy formation and evolution. We show that the theoretical SFRFs are well described by a double power law functional form and its redshift evolution is approximated with high accuracy by a pure evolution of the typical SFR. We find good agreement between model predictions and the high-SFR end of the SFRF, when the observational errors on the SFR are taken into account. However, the observational SFRF is characterised by a double peaked structure, which is absent in its theoretical counterparts. At z>1.0 the observed SFRF shows a relevant density evolution, which is not reproduced by SAMs, due to the well known overprediction of intermediate mass galaxies at z~2. The agreement at the low-SFR end is poor: all models overpredict the space density of SFR~1 Msun/yr and no model reproduces the double peaked shape of the observational SFRF. If confirmed by deeper IR observations, this discrepancy will provide a key constraint on theoretical modelling of star formation and stellar feedback.Comment: 12 pages, 4 figures and 3 table. Accepted for publication by MNRAS - updated reference

Semi-analytic galaxy formation in coupled dark energy cosmologies

Among the possible alternatives to the standard cosmological model ($\Lambda$CDM), coupled Dark Energy models postulate that Dark Energy (DE), seen as a dynamical scalar field, may interact with Dark Matter (DM), giving rise to a "fifth-force", felt by DM particles only. In this paper, we study the impact of these cosmologies on the statistical properties of galaxy populations by combining high-resolution numerical simulations with semi-analytic models (SAM) of galaxy formation and evolution. New features have been implemented in the reference SAM in order to have it run self-consistently and calibrated on these cosmological simulations. They include an appropriate modification of the mass temperature relation and of the baryon fraction in DM haloes, due to the different virial scalings and to the gravitational bias, respectively. Our results show that the predictions of our coupled-DE SAM do not differ significantly from theoretical predictions obtained with standard SAMs applied to a reference $\Lambda$CDM simulation, implying that the statistical properties of galaxies provide only a weak probe for these alternative cosmological models. On the other hand, we show that both galaxy bias and the galaxy pairwise velocity distribution are sensitive to coupled DE models: this implies that these probes might be successfully applied to disentangle among quintessence, $f(R)$-Gravity and coupled DE models.Comment: 8 pages, 1 Table, 5 Figures, MNRAS submitte

The Evolution of Sizes and Specific Angular Momenta in Hierarchical Models of Galaxy Formation and Evolution

We extend our previous work focused at $z\sim0$, studying the redshift evolution of galaxy dynamical properties using the state-of-the-art semi-analytic model GAEA: we show that the predicted size-mass relation for disky/star forming and quiescent galaxies is in good agreement with observational estimates, up to $z\sim2$. Bulge dominated galaxies have sizes that are offset low with respect to observational estimates, mainly due to our implementation of disk instability at high redshift. At large masses, both quiescent and bulge dominated galaxies have sizes smaller than observed. We interpret this as a consequence of our most massive galaxies having larger gas masses than observed, and therefore being more affected by dissipation. We argue that a proper treatment of quasar driven winds is needed to alleviate this problem. Our model compact galaxies have number densities in agreement with observational estimates and they form most of their stars in small and low angular momentum high-$z$ halos. GAEA predicts that a significant fraction of compact galaxies forming at high-$z$ is bound to merge with larger structures at lower redshifts: therefore they are not the progenitors of normal-size passive galaxies at $z=0$. Our model also predicts a stellar-halo size relation that is in good agreement with observational estimates. The ratio between stellar size and halo size is proportional to the halo spin and does not depend on stellar mass but for the most massive galaxies, where AGN feedback leads to a significant decrease of the retention factor (from about 80 per cent to 20 per cent).Comment: Accepted for publication in MNRAS, 17 pages, 11 figure

The active and passive populations of Extremely Red Objects

[abridged] The properties of galaxies with the reddest observed R-K colors (Extremely Red Objects, EROs), including their apparent division into passive and obscured active objects with roughly similar number densities, are a known challenge for models of galaxy formation. We produce mock catalogues generated by interfacing the predictions of the semi-analytical MORGANA model for the evolution of galaxies in a Lambda-CDM cosmology with the spectro-photometric + radiative transfer code GRASIL and Infrared (IR) template library to show that the model correctly reproduces number counts, redshift distributions and active fractions of R-K>5 sources. We test the robustness of our results against different dust attenuations and, most importantly, against the inclusion of TP-AGB stars in Simple Stellar Populations used to generate galaxy spectra, and find that the inclusion of TP-AGBs has a relevant effect, in that it allows to increase by a large factor the number of very red active objects at all color cuts. We find that though the most passive and the most obscured active galaxies have a higher probability of being selected as EROs, many EROs have intermediate properties and the population does not show bimodality in specific star formation rate (SSFR). We predict that deep observations in the Far-IR, from 100 to 500 micron, are the most efficient way to constrain the SSFR of these objects; we give predictions for future Herschel observations. Finally, we test whether a simple evolutionary sequence for the formation of z=0 massive galaxies, going through a sub-mm-bright phase and then a ERO phase, are typical in this galaxy formation model. We find that this sequence holds for ~25 per cent of z=0 massive galaxies, while the model typically shows a more complex connection between sub-mm, ERO and massive galaxies. [abridged]Comment: 14 pages, 12 figures, MNRAS in pres