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

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

Strong stellar-driven outflows shape the evolution of galaxies at cosmic dawn

We study galaxy mass assembly and cosmic star formation rate (SFR) at high-redshift (z$\gt$4), by comparing data from multiwavelength surveys with predictions from the GAlaxy Evolution and Assembly (GAEA) model. GAEA implements a stellar feedback scheme partially based on cosmological hydrodynamical simulations, that features strong stellar driven outflows and mass-dependent timescale for the re-accretion of ejected gas. In previous work, we have shown that this scheme is able to correctly reproduce the evolution of the galaxy stellar mass function (GSMF) up to $z\sim3$. We contrast model predictions with both rest-frame Ultra-Violet (UV) and optical luminosity functions (LF), which are mostly sensible to the SFR and stellar mass, respectively. We show that GAEA is able to reproduce the shape and redshift evolution of both sets of LFs. We study the impact of dust on the predicted LFs and we find that the required level of dust attenuation is in qualitative agreement with recent estimates based on the UV continuum slope. The consistency between data and model predictions holds for the redshift evolution of the physical quantities well beyond the redshift range considered for the calibration of the original model. In particular, we show that GAEA is able to recover the evolution of the GSMF up to z$\sim$7 and the cosmic SFR density up to z$\sim$10.Comment: 6 pages, 2 figures, accepted on ApJ Letter

Isolated galaxies in hierarchical galaxy formation models - present-day properties and environmental histories

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 observational 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.Comment: 15 pages, 13 figures, minor changes in the text, accepted for publication by MNRA

Structural and Dynamical Properties of Galaxies in a Hierarchical Universe: Sizes and Specific Angular Momenta

We use a state-of-the-art semi-analytic model to study the size and the specific angular momentum of galaxies. Our model includes a specific treatment for the angular momentum exchange between different galactic components. Disk scale radii are estimated from the angular momentum of the gaseous/stellar disk, while bulge sizes are estimated assuming energy conservation. The predicted size--mass and angular momentum--mass relations are in fair agreement with observational measurements in the local Universe, provided a treatment for gas dissipation during major mergers is included. Our treatment for disk instability leads to unrealistically small radii of bulges formed through this channel, and predicts an offset between the size--mass relations of central and satellite early-type galaxies, that is not observed. The model reproduces the observed dependence of the size--mass relation on morphology, and predicts a strong correlation between specific angular momentum and cold gas content. This correlation is a natural consequence of galaxy evolution: gas-rich galaxies reside in smaller halos, and form stars gradually until present day, while gas-poor ones reside in massive halos, that formed most of their stars at early epochs, when the angular momentum of their parent halos is low. The dynamical and structural properties of galaxies can be strongly affected by a different treatment for stellar feedback, as this would modify their star formation history. A higher angular momentum for gas accreted through rapid mode does not affect significantly the properties of massive galaxies today, but has a more important effect on low-mass galaxies at higher redshift.Comment: 26 pages, 14 figures, 4 appendices. Accepted for publication in MNRA

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 Role of Black Hole Feedback on Size and Structural Evolution in Massive Galaxies

We use cosmological hydrodynamical simulations to investigate the role of feedback from accreting black holes on the evolution of sizes, compactness, stellar core density and specific star-formation of massive galaxies with stellar masses of $M_{star} > 10^{10.9} M_{\odot}$. We perform two sets of cosmological zoom-in simulations of 30 halos to z=0: (1) without black holes and Active Galactic Nucleus (AGN) feedback and (2) with AGN feedback arising from winds and X-ray radiation. We find that AGN feedback can alter the stellar density distribution, reduce the core density within the central 1 kpc by 0.3 dex from z=1, and enhance the size growth of massive galaxies. We also find that galaxies simulated with AGN feedback evolve along similar tracks to those characterized by observations in specific star formation versus compactness. We confirm that AGN feedback plays an important role in transforming galaxies from blue compact galaxies into red extended galaxies in two ways: (1) it effectively quenches the star formation, transforming blue compact galaxies into compact quiescent galaxies and (2) it also removes and prevents new accretion of cold gas, shutting down in-situ star formation and causing subsequent mergers to be gas-poor or mixed. Gas poor minor mergers then build up an extended stellar envelope. AGN feedback also puffs up the central region through the fast AGN driven winds as well as the slow expulsion of gas while the black hole is quiescent. Without AGN feedback, large amounts of gas accumulate in the central region, triggering star formation and leading to overly massive blue galaxies with dense stellar cores.Comment: 13 pages, 7 figures, Accepted for publication in Ap

Variations of the initial mass function in semi-analytical models: implications for the mass assembly and the chemical enrichment of galaxies in the GAEA model

In this work, we investigate the implications of the Integrated Galaxy-wide stellar Initial Mass Function (IGIMF) approach in the framework of the semi-analytic model GAEA (GAlaxy Evolution and Assembly), which features a detailed treatment of chemical enrichment and stellar feedback. The IGIMF provides an analytic description of the dependence of the stellar IMF shape on the rate of star formation in galaxies. We find that our model with a universal IMF predicts a rather flat [$\alpha$/Fe]-stellar mass relation. The model assuming the IGIMF, instead, is able to reproduce the observed increase of $\alpha$-enhancement with stellar mass, in agreement with previous studies. This is mainly due to the fact that massive galaxies are characterized by larger star formation rates at high-redshift, leading to stronger $\alpha$-enhancement with respect to low-mass galaxies. At the same time, the IGIMF hypothesis does not affect significantly the trend for shorter star formation timescales for more massive galaxies. We argue that in the IGIMF scenario the [$\alpha$/Fe] ratios are good tracers of the highest star formation events. The final stellar masses and mass-to-light-ratio of our model massive galaxies are larger than those estimated from the synthetic photometry assuming a universal IMF, providing a self-consistent interpretation of similar recent results, based on dynamical analysis of local early type galaxies.Comment: 14 pages, 8 figures, 2 tables, submitted to MNRA

Cosmological simulations of black hole growth: AGN luminosities and downsizing

In this study, we present a detailed, statistical analysis of black hole growth and the evolution of active galactic nuclei (AGN) using cosmological hydrodynamic simulations run down to $z=0$. The simulations self-consistently follow radiative cooling, star formation, metal enrichment, black hole growth and associated feedback processes from both supernovae typeII/Ia and AGN. We consider two simulation runs, one with a large co-moving volume of $(500\ \mathrm{Mpc})^3$and one with a smaller volume of$(68\ \mathrm{Mpc})^3$but with a by a factor of almost 20 higher mass resolution. Consistently with previous results, our simulations can widely match observed black hole properties of the local Universe. Furthermore, our simulations can successfully reproduce the evolution of the bolometric AGN luminosity function for both the low-luminosity and the high-luminosity end up to$z=3.0$. In addition, the smaller but higher resolution run is able to match the observational data of the low bolometric luminosity end at higher redshifts$z=3-4$. We also perform a direct comparison with the observed soft and hard X-ray luminosity functions of AGN, including an empirical correction for a torus-level obscuration, and find a similarly good agreement. These results nicely demonstrate that the observed "anti-hierarchical" trend in the AGN number density evolution (i.e. the number densities of luminous AGN peak at higher redshifts than those of faint AGN) is self-consistently predicted by our simulations. Implications of this downsizing behaviour on active black holes, their masses and Eddington-ratios are discussed. Overall, the downsizing behaviour in the AGN number density as a function of redshift can be mainly attributed to the evolution of the gas density in the resolved vicinity of a (massive) black hole. (shortened)Comment: 24 pages, 15 figures, 1 table, accepted for publication in MNRAS, the analysis is updated using a simulation run with a cosmological volume of (500Mpc)^3 containing 2*1,564^3 particle