arXiv subjects

Fabio Fontanot

Publications and source records attributed to Fabio Fontanot.

At least 91 records · Page 5Linked to original sources

arXiv2016

AGN feedback and the origin of the $α$ enhancement in early type galaxies - insights from the GAEA model

We take advantage of our recently published model for Galaxy Evolution and Assembly (GAEA) to study the origin of the observed correlation between [$α$/Fe] and galaxy stellar mass. In particular, we analyse the role of radio mode AGN feedback, that recent work has identified as a crucial ingredient to reproduce observations. In GAEA, this process introduces the observed trend of star formation histories extending over shorter time-scales for more massive galaxies, but does not provide a sufficient condition to reproduce the observed $α$ enhancements of massive galaxies. In the framework of our model, this is possible only assuming that any residual star formation is truncated for galaxies more massive than $10^{10.5}\,{\rm M}_{\odot}$. This results, however, in even shorter star formation time-scales for the most massive galaxies, that translate in total stellar metallicities significantly lower than observed. Our results demonstrate that (i) trends of [$α$/Fe] ratios cannot be simply converted in relative time-scale indicators, and (ii) AGN feedback cannot explain alone the positive correlation between [$α$/Fe] and galaxy mass/velocity dispersion. Reproducing simultaneously the mass-metallicity relation and the $α$ enhancements observed poses a challenge for hierarchical models, unless more exotic solutions are adopted such as metal-rich winds or a variable IMF.

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arXiv2016

HI-selected Galaxies in Hierarchical Models of Galaxy Formation and Evolution

In this work, we study the basic statistical properties of HI-selected galaxies extracted from six different semi-analytic models, all run on the same cosmological N-body simulation. One model includes an explicit treatment for the partition of cold gas into atomic and molecular hydrogen. All models considered agree nicely with the measured HI mass function in the local Universe, with the measured scaling relations between HI and galaxy stellar mass, and with the predicted 2-point correlation function for HI rich galaxies. One exception is given by one model that predicts very little HI associated with galaxies in haloes above 10^12 Msun: we argue this is due to a too efficient radio-mode feedback for central galaxies, and to a combination of efficient stellar feedback and instantaneous stripping of hot gas for satellites. We demonstrate that treatment of satellite galaxies introduces large uncertainties at low HI masses. While models assuming non instantaneous stripping of hot gas tend to form satellite galaxies with HI masses slightly smaller than those of centrals with the same stellar mass, instantaneous gas stripping does not translate necessarily in lower HI masses. In fact, the adopted stellar feedback and star formation affect the satellites too. We analyze the relation between HI content and spin of simulated haloes: low spin haloes tend to host HI poor galaxies while high spin haloes are populated by galaxies in a wide range of HI mass. In our simulations, this is due to a correlation between the initial gas disk size and the halo spin.

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arXiv2016

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 [$α$/Fe]-stellar mass relation. The model assuming the IGIMF, instead, is able to reproduce the observed increase of $α$-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 $α$-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 [$α$/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.

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arXiv2016

The Spectral Slope and Escape Fraction of Bright Quasars at $z \sim 3.8$: the Contribution to the Cosmic UV Background

We use a sample of 1669 QSOs ($r<20.15$, $3.6<z<4.0$) from the BOSS survey to study the intrinsic shape of their continuum and the Lyman continuum photon escape fraction (f$_{esc}$), estimated as the ratio between the observed flux and the expected intrinsic flux (corrected for the intergalactic medium absorption) in the wavelength range 865-885 Å rest-frame. Modelling the intrinsic QSO continuum shape with a power-law, $F_λ\proptoλ^{-γ}$, we find a median $γ=1.30$ (with a dispersion of $0.38$, no dependence on the redshift and a mild intrinsic luminosity dependence) and a mean f$_{esc}=0.75$ (independent of the QSO luminosity and/or redshift). The f$_{esc}$ distribution shows a peak around zero and a long tail of higher values, with a resulting dispersion of $0.7$. If we assume for the QSO continuum a double power-law shape (also compatible with the data) with a break located at $λ_{\rm br}=1000$ Å and a softening $Δγ=0.72 $ at wavelengths shorter than $λ_{\rm br}$, the mean f$_{esc}$ rises to $=0.82$. Combining our $γ$ and f$_{esc}$ estimates with the observed evolution of the AGN luminosity function (LF) we compute the AGN contribution to the UV ionizing background (UVB) as a function of redshift. AGN brighter than one tenth of the characteristic luminosity of the LF are able to produce most of it up $z\sim 3$, if the present sample is representative of their properties. At higher redshifts a contribution of the galaxy population is required. Assuming an escape fraction of Lyman continuum photons from galaxies between $5.5$ and $7.6\%$, independent of the galaxy luminosity and/or redshift, a remarkably good fit to the observational UVB data up to $z\sim 6$ is obtained. At lower redshift the extrapolation of our empirical estimate agrees well with recent UVB observations, dispelling the so-called Photon Underproduction Crisis.

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arXiv2016

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.

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arXiv2015

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.

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arXiv2015

On the dependence of galaxy morphologies on galaxy mergers

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 components. 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.

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arXiv2015

From Discs to Bulges: effect of mergers on the morphology of galaxies

We study the effect of mergers on the morphology of galaxies by means of the simulated merger tree approach first proposed by Moster et al. This method combines N-body cosmological simulations and semi-analytic techniques to extract realistic initial conditions for galaxy mergers. These are then evolved using high resolution hydrodynamical simulations, which include dark matter, stars, cold gas in the disc and hot gas in the halo. We show that the satellite mass accretion is not as effective as previously thought, as there is substantial stellar stripping before the final merger. The fraction of stellar disc mass transferred to the bulge is quite low, even in the case of a major merger, mainly due to the dispersion of part of the stellar disc mass into the halo. We confirm the findings of Hopkins et al., that a gas rich disc is able to survive major mergers more efficiently. The enhanced star formation associated with the merger is not localised to the bulge of galaxy, but a substantial fraction takes place in the disc too. The inclusion of the hot gas reservoir in the galaxy model contributes to reducing the efficiency of bulge formation. Overall, our findings suggest that mergers are not as efficient as previously thought in transforming discs into bulges. This possibly alleviates some of the tensions between observations of bulgeless galaxies and the hierarchical scenario for structure formation.

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arXiv2015

nIFTy Cosmology: Comparison of Galaxy Formation Models

We present a comparison of 14 galaxy formation models: 12 different semi-analytical models and 2 halo-occupation distribution models for galaxy formation based upon the same cosmological simulation and merger tree information derived from it. The participating codes have proven to be very successful in their own right but they have all been calibrated independently using various observational data sets, stellar models, and merger trees. In this paper we apply them without recalibration and this leads to a wide variety of predictions for the stellar mass function, specific star formation rates, stellar-to- halo mass ratios, and the abundance of orphan galaxies. The scatter is much larger than seen in previous comparison studies primarily because the codes have been used outside of their native environment within which they are well tested and calibrated. The purpose of the `nIFTy comparison of galaxy formation models' is to bring together as many different galaxy formation modellers as possible and to investigate a common approach to model calibration. This paper provides a unified description for all participating models and presents the initial, uncalibrated comparison as a baseline for our future studies where we will develop a common calibration framework and address the extent to which that reduces the scatter in the model predictions seen here.

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arXiv2015

Semi-analytic galaxy formation in coupled dark energy cosmologies

Among the possible alternatives to the standard cosmological model ($Λ$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 $Λ$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.

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arXiv2014

Semi-Analytic Galaxy Formation in Massive Neutrino Cosmologies

The constraints on neutrino masses led to the revision of their cosmological role, since the existence of a cosmological neutrino background is a clear prediction of the standard cosmological model. In this paper, we study the impact of such background on the spatial distribution of both Dark Matter (DM) and galaxies, by coupling $N$-body numerical simulations with semi-analytic models (SAMs) of galaxy formation. Cosmological simulations including massive neutrinos predict a slower evolution of DM perturbations with respect to the $Λ$CDM runs with the same initial conditions and a suppression on the matter power spectrum on small and intermediate scales, thus impacting on the predicted properties of galaxy populations. We explicitly show that most of these deviations are driven by the different $σ_8$ predicted for cosmologies including a massive neutrino background. We conclude that independent estimates of $σ_8$ are needed, in order to unambiguously characterise the effect of this background on the growth of structures. Galaxy properties alone are a weak tracer of deviations with respect to the $Λ$CDM run, but their combination with the overall matter distribution at all scales allows to disentangle between different cosmological models. Moreover, these deviations go on opposite direction with respect to competing models like modified gravity, thus weakening any detectable cosmological signal. Given the ubiquitous presence of a neutrino background, these effects have to be taken into account in future missions aimed at constraining the properties of the "Dark" components of the Universe.

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arXiv2014

The definition of environment and its relation to the quenching of galaxies at z=1-2 in a hierarchical Universe

A well calibrated method to describe the environment of galaxies at all redshifts is essential for the study of structure formation. Such a calibration should include well understood correlations with halo mass, and the possibility to identify galaxies which dominate their potential well (centrals), and their satellites. Focusing on z = 1 and 2 we propose a method of environmental calibration which can be applied to the next generation of low to medium resolution spectroscopic surveys. Using an up-to-date semi-analytic model of galaxy formation, we measure the local density of galaxies in fixed apertures on different scales. There is a clear correlation of density with halo mass for satellite galaxies, while a significant population of low mass centrals is found at high densities in the neighbourhood of massive haloes. In this case the density simply traces the mass of the most massive halo within the aperture. To identify central and satellite galaxies, we apply an observationally motivated stellar mass rank method which is both highly pure and complete, especially in the more massive haloes where such a division is most meaningful. Finally we examine a test case for the recovery of environmental trends: the passive fraction of galaxies and its dependence on stellar and halo mass for centrals and satellites. With careful calibration, observationally defined quantities do a good job of recovering known trends in the model. This result stands even with reduced redshift accuracy, provided the sample is deep enough to preserve a wide dynamic range of density.

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arXiv2014

Variations of the Initial Mass Function in Semi-Analytical models

Deviations from a universal, MW-like, Stellar Initial Mass Function (IMF) have been reported for distant galaxies, although the physical reason behind the observed variations is still matter of ongoing debate. In this paper, we present an exploratory study to assess the impact of the proposed IMF evolution on the statistical galaxy properties, as predicted by the Semi-Analytical model of galaxy formation and evolution MORGANA. In particular, we test different dependencies for the IMF shape, as a function of both model galaxy properties (such as star formation rate, velocity dispersion or stellar mass) and environment, and compare the predicted stellar mass functions and star formation rate functions with reference runs at fixed IMF. In most cases, MORGANA predictions show deviations of the order of a few tenths of dex with respect to a run assuming an Universal Kroupa IMF. Among the proposed IMF variations, an increasing Top-Heavy IMF at increasing star formation rates has the largest impact on predicted galaxy properties, while most of the models assuming an increasing Bottom-Heavy IMF at higher masses/velocity dispersion lead to variations in galaxy properties that are of the same order as the uncertainty on the mass and star formation rate determination. By comparing the predicted galaxy stellar mass functions, we conclude that the study of the high-mass end can provide useful constraints to disentangle models assuming an increasing Top-Heavy IMF in high star forming or Bottom-Heavy IMF in massive systems.

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arXiv2014

A semi-analytic model comparison: testing cooling models against hydrodynamical simulations

We compare predictions of cooled masses and cooling rates from three stripped-down Semi-Analytic Models (SAMs) of galaxy formation with the results of N-body+SPH simulations with gas particle mass of 3.9x10^6 Msun, where radiative cooling of a gas of primordial composition is implemented. We also run a simulation where cooling is switched on at redshift ~2, in order to test cooling models in a regime in which their approximations are expected to be valid. We confirm that cooling models implemented in SAMs are able to predict the amount of cooled mass at z=0 to within ~20 per cent. However, some relevant discrepancies are found. (i) When the contribution from poorly resolved halos is subtracted out, SAMs tend to under-predict by ~30 per cent the mass that cools in the infall-dominated regime. (ii) At large halo masses SAMs tend to over-predict cooling rates, though the numerical result may be affected by the use of SPH. (iii) As found in our previous work, cooling rates are found to be significantly affected by model details: simulations disfavour models with large cores and with quenching of cooling at major mergers. (iv) When cooling is switched on at z~2, cold gas accumulates very quickly in the simulated halos. This accumulation is reproduced by SAMs with varying degrees of accuracy.

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arXiv2014

Environmental dependence of bulge-dominated galaxy sizes in hierarchical models of galaxy formation. Comparison with the local Universe

We compare state-of-the-art semi-analytic models of galaxy formation as well as advanced sub-halo abundance matching models with a large sample of early-type galaxies from SDSS at z < 0.3. We focus our attention on the dependence of median sizes of central galaxies on host halo mass. The data do not show any difference in the structural properties of early-type galaxies with environment, at fixed stellar mass. All hierarchical models considered in this work instead tend to predict a moderate to strong environmental dependence, with the median size increasing by a factor of about 1.5-3 when moving from low to high mass host haloes. At face value the discrepancy with the data is highly significant, especially at the cluster scale, for haloes above log Mhalo > 14. The convolution with (correlated) observational errors reduces some of the tension. Despite the observational uncertainties, the data tend to disfavour hierarchical models characterized by a relevant contribution of disc instabilities to the formation of spheroids, strong gas dissipation in (major) mergers, short dynamical friction timescales, and very short quenching timescales in infalling satellites. We also discuss a variety of additional related issues, such as the slope and scatter in the local size-stellar mass relation, the fraction of gas in local early-type galaxies, and the general predictions on satellite galaxies.

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arXiv2013

On the Evolution of the Cosmic Ionizing Background

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.

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arXiv2013

Semi-analytic galaxy formation in f(R)-gravity cosmologies

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.

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arXiv2013

The Hierarchical Origins of Observed Galaxy Morphology

Galaxies grow primarily via accretion-driven star formation in discs and merger-driven growth of bulges. These processes are implicit in semi-analytical models of galaxy formation, with bulge growth in particular relating directly to the hierarchical build-up of halos and their galaxies. In this paper, we consider several implementations of two semi-analytical models. Focusing on implementations in which bulges are formed during mergers only, we examine the fractions of elliptical galaxies and both passive and star-forming disk galaxies as functions of stellar and halo mass, for central and satellite systems. This is compared to an observational cross-matched SDSS+RC3 z ~ 0 sample of galaxies with accurate visual morphological classifications and M_{stellar} > 10^10.5 M_{sol}. The models qualitatively reproduce the observed increase of elliptical fraction with stellar mass, and with halo mass for central galaxies, supporting the idea that observed ellipticals form during major mergers. However, the overall elliptical fraction produced by the models is much too high compared with the z ~ 0 data. Since the "passive" -- i.e. non-star-forming -- fractions are approximately reproduced, and since the fraction which are star-forming disc galaxies is also reproduced, the problem is that the models overproduce ellipticals at the expense of passive S0 and spiral galaxies. Bulge-growth implementations (tuned to reproduce simulations) which allow the survival of residual discs in major mergers still destroy too much of the disc. Increasing the lifetime of satellites, or allowing significant disc regrowth around merger remnants, merely increases the fraction of star-forming disc galaxies. Instead, it seems necessary to reduce the mass ratios of merging galaxies, so that most mergers produce modest bulge growth in disc-galaxy remnants instead of ellipticals. [Abridged]

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