arXiv subjects

Fabio Fontanot

Publications and source records attributed to Fabio Fontanot.

119 records · Page 7Linked to original sources

arXiv2009

Faint Lyman-Break galaxies as a crucial test for galaxy formation models

It has recently been shown that galaxy formation models within the LambdaCDM cosmology predict that, compared to the observed population, small galaxies (with stellar masses < 10^{11} M_sun) form too early, are too passive since z ~ 3 and host too old stellar populations at z=0. We then expect an overproduction of small galaxies at z > 4 that should be visible as an excess of faint Lyman-break galaxies. To check whether this excess is present, we use the MORGANA galaxy formation model and GRASIL spectro-photometric + radiative transfer code to generate mock catalogues of deep fields observed with HST-ACS. We add observational noise and the effect of Lyman-alpha emission, and perform color-color selections to identify Lyman-break galaxies. The resulting mock candidates have plausible properties that closely resemble those of observed galaxies. We are able to reproduce the evolution of the bright tail of the luminosity function of Lyman-break galaxies (with a possible underestimate of the number of the brightest i-dropouts), but uncertainties and degeneracies in dust absorption parameters do not allow to give strong constraints to the model. Besides, our model shows a clear excess with respect to observations of faint Lyman-break galaxies, especially of z_{850} ~ 27 V-dropouts at z ~ 5. We quantify the properties of these "excess" galaxies and discuss the implications: these galaxies are hosted in dark matter halos with circular velocities in excess of 100 km s^{-1}, and their suppression may require a deep re-thinking of stellar feedback processes taking place in galaxy formation.

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arXiv2009

The Many Manifestations of Downsizing: Hierarchical Galaxy Formation Models confront Observations

[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 "bottom-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]

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arXiv2009

The Correlation of Star Formation Quenching with Internal Galaxy Properties and Environment

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 Catalog of Yang and collaborators, which is based on the Sloan Digital Sky Survey. Data from the Galaxy Evolution Explorer (GALEX) 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 AGN 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 {\em satellite overquenching problem} is caused by the too-rapid stripping of the satellites' hot gas halos, which leads to rapid strangulation of star formation.

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arXiv2008

Evaluating and Improving Semi-analytic modelling of Dust in Galaxies based on Radiative Transfer Calculations

The treatment of dust attenuation is crucial in order to compare the predictions of galaxy formation models with multiwavelength observations. Most past studies have either used simple analytic prescriptions or else full radiative transfer (RT) calculations. Here, we couple star formation histories and morphologies predicted by the semi-analytic galaxy formation model MORGANA with RT calculations from the spectrophotometric and dust code GRASIL to create a library of galaxy SEDs from the UV/optical through the far Infrared, and compare the predictions of the RT calculations with analytic prescriptions. We consider a low and high redshift sample, as well as an additional library constructed with empirical, non-cosmological star formation histories and simple (pure bulge or disc) morphologies. Based on these libraries, we derive fitting formulae for the effective dust optical depth as a function of galaxy physical properties such as metallicity, gas mass, and radius. We show that such fitting formulae can predict the V-band optical depth with a scatter smaller than 0.4 dex for both the low and high redshift samples, but that there is a large galaxy-to-galaxy scatter in the shapes of attenuation curves, probably due to geometrical variations, which our simple recipe does not capture well. However, our new recipe provides a better approximation to the GRASIL results at optical wavelength than standard analytic prescriptions from the literature, particularly at high redshift.

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arXiv2008

Expected z>5 QSO number counts in large area deep near-infrared surveys

The QSO luminosity function at z>5 provides strong constraints on models of joint evolution of QSO and their hosts. However, these observations are challenging because the low space densities of these objects necessitate surveying of large areas, in order to obtain statistically meaningful samples, while at the same time cosmological redshifting and dimming means that rather deep Near Infrared (NIR) imaging must be carried out. Several upcoming and proposed facilities with wide-field NIR imaging capabilities will open up this new region of parameter space. In this paper we present predictions for the expected number counts of z>5 QSOs, based on simple empirical and semi-empirical models of QSO evolution, as a function of redshift, depth and surveyed area. We compute the evolution of observed-frame QSO magnitudes and colors in a representative photometric system covering the wavelength range 550nm<lambda<1800nm, and combine this information with different estimates for the evolution of the QSO luminosity function. We conclude that planned ground-based surveys such as Pan-STARRS and VISTA should be able to detect a large number of luminous QSOs up to z<7.5, but that space-based missions such as EUCLID (formerly SPACE/DUNE) or SNAP are probably required in order to obtain substantial samples at higher redshift. We also use our models to predict the expected number counts for future X-ray space missions (such as XEUS and Constellation-X), and show that because of their small field-of-view, these telescopes are unlikely to discover significant numbers of AGN at very high redshift. However, X-ray follow-up of objects detected at longer wavelength will be an important means of confirming their identity as AGN and constraining obscuration.

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arXiv2007

Reproducing the assembly of massive galaxies within the hierarchical cosmogony

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 micron-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.

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arXiv2006

The luminosity function of high-redshift QSOs

We measure the luminosity function of QSOs in the redshift range 3.5 < z < 5.2 for the absolute magnitude interval -21 < M_{145} < -28. Suitable criteria are defined to select faint QSOs in the GOODS fields, checking their effectiveness and completeness in detail. The confirmed sample of faint QSOs is compared with a brighter one derived from the SDSS. Using a Monte-Carlo technique we estimate the properties of the luminosity function. Our results show that models based on pure density evolution show better agreement with observation than models based on pure luminosity evolution, even if a different break magnitude with respect to z ~ 2.1 is required at 3.5 < z < 5.2. According to our modeling a faint-end slope steeper than low-redshift observations is required to reproduce the data, moreover models with a steep bright-end slope score a higher probability than models with a bright-end flattening. Determining the faint-end of the luminosity function at these redshifts provides important constraints on models of the joint evolution of galaxies and AGNs.

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arXiv2006

The MORGANA model for the rise of galaxies and active nuclei

We present the MOdel for the Rise of GAlaxies aNd Active nuclei (MORGANA). Starting from the merger trees of dark matter halos and a model for the evolution of substructure within the halos, the complex physics of baryons is modeled with a set of state-of-the-art models that describe the mass, metal and energy flows between the various components and phases of a galaxy. The processes of shock-heating and cooling, star formation, feedback, galactic winds and super-winds, accretion onto BHs 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 multi-phase model by Monaco (2004a). The increased level of sophistication allows to move from a phenomenological description of gas physics, based on simple scalings with the depth of the DM halo potential, toward a fully physically motivated one. 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 cutoff 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 dark matter halos are assumed to have a very low concentration, (iii) the mass function of HI 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. (ABRIDGED)

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arXiv2006

Diffuse stellar component in galaxy clusters and the evolution of the most massive galaxies at z<~1

The high end of the stellar mass function of galaxies is observed to have little evolution since z~1. This represents a stringent constraint for merger--based models, aimed at explaining the evolution of the most massive galaxies in the concordance LambdaCDM cosmology. In this Letter we show that it is possible to remove the tension between the above observations and model predictions by allowing a fraction of stars to be scattered to the Diffuse Stellar Component (DSC) of galaxy clusters at each galaxy merger, as recently suggested by the analysis of N-body hydrodynamical simulations. To this purpose, we use the MORGANA model of galaxy formation in a minimal version, in which gas cooling and star formation are switched off after z=1. In this way, any predicted evolution of the galaxy stellar mass function is purely driven by mergers. We show that, even in this extreme case, the predicted degree of evolution of the high end of the stellar mass function is larger than that suggested by data. Assuming instead that a significant fraction, ~30 per cent, of stars are scattered in the DSC at each merger event, leads to a significant suppression of the predicted evolution, in better agreement with observational constraints, while providing a total amount of DSC in clusters which is consistent with recent observational determinations.

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arXiv2006

The effect of stellar feedback and quasar winds on the AGN population

In order to constrain the physical processes that regulate and downsize the AGN population, the predictions of the MOdel for the Rise of GAlaxies aNd Active nuclei (MORGANA) are compared to luminosity functions (LFs) of AGNs in the optical, soft X-ray and hard X-ray bands, to the local BH-bulge mass relation, and to the observed X-ray number counts and background. We also give predictions on the accretion rate of AGNs in units of the Eddington rate and on the BH--bulge relation expected at high redshift. We find that it is possible to reproduce the downsizing of AGNs within the hierarchical LambdaCDM cosmogony, and that the most likely responsible for this downsizing is the stellar kinetic feedback that arises in star-forming bulges as a consequence of the high level of turbulence and leads to a massive removal of cold gas in small elliptical galaxies. At the same time, to obtain good fits to the number of bright quasars we need to require that quasar-triggered galactic winds self-limit the accretion onto BHs. In all cases, the predicted BH--bulge relation steepens considerably with respect to the observed one at bulge masses <10^{11} Msun; this problem is related to a known excess in the predicted number of small bulges, common to most similar models, so that the reproduction of the correct number of faint AGNs is done at the cost of underestimating their BH masses. This highlights an insufficient downsizing of elliptical galaxies, and hints for another feedback mechanism able to act on the compact discs that form and soon merge at high redshift. The results of this paper reinforce the need for direct investigations of the feedback mechanisms in active galaxies, that will be possible with the next generation of astronomical telescopes from sub-mm to X-rays.

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arXiv2003

Recovering the Initial Condition of our Local Universe from NOG and PSCz Catalogues

We apply the ZTRACE algorithm to the optical NOG and infra-red PSCz galaxy catalogues to reconstruct the pattern of primordial fluctuations that have generated our local Universe. We check that the density fields traced by the two catalogues are well correlated, and consistent with a linear relation (either in $δ$ or in log(1+$δ$)) with relative bias (of NOG with respect to PSCz) b_rel = 1.1 $\pm$ 0.1. The relative bias relation is used to fill the optical zone of avoidance at |b| < 20$^\circ$ using the PSCz galaxy density field. We perform extensive testing on simulated galaxy catalogues to optimize the reconstruction. The quality of the reconstruction is predicted to be good at large scales, up to a limiting wavenumber k_lim $\simeq$ 0.4 h/Mpc beyond which all information is lost. We find that the improvement due to the denser sampling of the optical catalogue is compensated by the uncertainties connected to the larger zone of avoidance. The initial conditions reconstructed from the NOG catalogue are found (analogously to those from the PSCz) to be consistent with Gaussian paradigm. We use the reconstructions to produce sets of initial conditions ready to be used for constrained simulations of our local Universe.

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