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Silva L.,National institute for astrophysics | Fontanot F.,Hits Heidelberger Institute For Theoretische Studien | Fontanot F.,Institute For Theoretische Physik | Granato G.L.,National institute for astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2012

A serious concern for semi-analytical galaxy formation models, aiming to simulate multiwavelength surveys and to thoroughly explore the model parameter space, is the extremely time-consuming numerical solution of the radiative transfer of stellar radiation through dusty media. To overcome this problem, we have implemented an artificial neural network (ANN) algorithm in the radiative transfer code grasil, in order to significantly speed up the computation of the infrared (IR) spectral energy distribution (SED). The ANN we have implemented is of general use, in that its input neurons are defined as those quantities effectively determining the shape of the IR SED. Therefore, the training of the ANN can be performed with any model and then applied to other models. We made a blind test to check the algorithm, by applying a net trained with a standard chemical evolution model (i.e. che_evo) to a mock catalogue extracted from the semi-analytic model morgana, and compared galaxy counts and evolution of the luminosity functions in several near-IR to sub-millimetre (sub-mm) bands, and also the spectral differences for a large subset of randomly extracted models. The ANN is able to excellently approximate the full computation, but with a gain in CPU time by ∼2 orders of magnitude. It is only advisable that the training covers reasonably well the range of values of the input neurons in the application. Indeed in the sub-mm at high redshift, a tiny fraction of models with some sensible input neurons out of the range of the trained net cause wrong answer by the ANN. These are extreme starbursting models with high optical depths, favourably selected by sub-mm observations, and are difficult to predict a priori. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source


Fontanot F.,National institute for astrophysics | Fontanot F.,Hits Heidelberger Institute For Theoretische Studien | Cristiani S.,National institute for astrophysics | Santini P.,National institute for astrophysics | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We study the evolution of the star formation rate function (SFRF) of massive (M * > 10 10M ⊙) 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 Great Observatories Origins Deep Survey-Multiwavelength Southern Infrared Catalog (GOODS-MUSIC) catalogue, which provides a suitable coverage of the spectral region from 0.3 to 24 m and either spectroscopic or photometric redshifts for each object. Individual SFRs have been obtained by combining ultraviolet and 24-m observations, when the latter were available. For all other sources a 'spectral energy distribution (SED) fitting' SFR estimate has been considered. We then define a stellar mass limited sample, complete in the M * > 10 10M ⊙ range and determine the SFRF using the 1/V max algorithm. We thus define simulated galaxy catalogues based on the predictions of three different state-of-the-art semi-analytical models (SAMs) of galaxy formation and evolution, and compare them with the observed SFRF. 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 (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 characterized 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. SAMs are thus able to reproduce the most intense SFR events observed in the GOODS-MUSIC sample and their redshift distribution. At the same time, the agreement at the low-SFR end is poor: all models overpredict the space density of SFR ~ 1M ⊙yr -1 and no model reproduces the double-peaked shape of the observational SFRF. If confirmed by deeper infrared observations, this discrepancy will provide a key constraint on theoretical modelling of star formation and stellar feedback. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source


Fontanot F.,National institute for astrophysics | Baldi M.,University of Bologna | Baldi M.,National institute for astrophysics | Baldi M.,National Institute of Nuclear Physics, Italy | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

Among the possible alternatives to the standard cosmological model (ΛCDM), 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 (SAMs) 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 cold dark matter (ΛCDM) 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. © 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Kannan R.,Max Planck Institute for Astronomy | Kannan R.,Massachusetts Institute of Technology | Maccio A.V.,Max Planck Institute for Astronomy | Fontanot F.,National institute for astrophysics | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

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 localized 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. © 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Fontanot F.,Hits Heidelberger Institute For Theoretische Studien | Fontanot F.,Institute For Theoretische Physik | Fontanot F.,National institute for astrophysics | Cristiani S.,National institute for astrophysics | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

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 ultraviolet (UV) luminosity function (LF) of Lyman-break galaxies (LBGs) at fixed Lyman continuum photon escape fraction (fesc) 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 fesc 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 characterized by fesc values 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 fesc values outside our proposed range bear relevant consequences on the nature of the astrophysical sources responsible for cosmic reionization and/or its build-up process. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

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