The interaction-driven starburst contribution to the cosmic star formation rate density

An increasing amount of observational evidence supports the notion that there are two modes of star formation: a quiescent mode in disk-like galaxies, and a starburst mode, which is generally interpreted as driven by merging. Using a semi-analytic model of galaxy formation, we derive the relative contribution to the cosmic star formation rate density of quiescently starforming and starburst galaxies, predicted under the assumption that starburst events are triggered by galaxy encounters (merging and fly-by kind) during their merging histories. We show that, within this framework, quiescently starforming galaxies dominate the cosmic star formation rate density at all redshifts. The contribution of the burst-dominated starforming galaxies increases with redshift, rising from <5% at low redshift (z5. We estimated that the fraction of the final (z=0) galaxy stellar mass which is formed through the burst component of star formation is ~10% for 10^10 M_\odot<M_*<10^11.5 M_\odot. Starburst galaxies, selected according to their distance from the galaxy main sequence, account for ~10% of the star formation rate density in the redshift interval 1.5<z<2.5, i.e. at the cosmic peak of the star formation activity.Comment: 11 pages, 8 figures. Accepted for publication in A&

The Building Up of the Black Hole Mass - Stellar Mass Relation

We derive the growth of SMBHs relative to the stellar content of their host galaxy predicted under the assumption of BH accretion triggered by galaxy encounters occurring during their merging histories. We show that, within this framework, the ratio Gamma=(M_BH/M_*)(z)/(M_BH/M_*)(z=0) between the Black Hole mass and the galactic stellar mass (normalized to the local value) depends on both BH mass and redshift. While the average value and the spread of Gamma(z) increase with z, such an effect is larger for massive BHs, reaching values Gamma=5 for massive Black Holes (M>10^9 M_{\odot}) at z>4, in agreement with recent observations of high-redshift QSOs; this is due to the effectiveness of interactions in triggering BH accretion in high-density environments at high redshifts. To test such a model against observations, we worked out specific predictions for sub-samples of the simulated galaxies corresponding to the different observational samples for which measurements of Gamma have been obtained. We found that for Broad Line AGNs at 1<z<2 values of Gamma=2 are expected, with a mild trend toward larger value for increasing BH mass. Instead, when we select from our Monte Carlo simulations only extremely gas rich, rapidly star forming galaxies at 2<z<3, we find low values 0.3<Gamma<1.5, consistent with recent observational findings on samples of sub-mm galaxies; in the framework of our model, these objects end up at z=0 in low-to-intermediate mass BHs (M<10^9 M_{\odot}), and they do not represent typical paths leading to local massive galaxies. The latter have formed preferentially through paths passing above the local M_*-M_BH relation. We discuss how the global picture emerging from the model is consistent with a downsizing scenario, where massive BHs accrete a larger fraction of their final mass at high redshifts z>4.Comment: 13 pages, 9 figures. Accepted to MNRA

The Epoch of Reionization in Warm Dark Matter Scenarios

In this paper we investigate how the Reionization process is affected by early galaxy formation in different cosmological scenarios. We use a semi-analytic model with suppressed initial power spectra to obtain the UV Luminosity Function in thermal Warm Dark Matter and sterile neutrino cosmologies. We retrace the ionization history of intergalactic medium with hot stellar emission only, exploiting fixed and variable photons escape fraction models (fesc). For each cosmology, we find an upper limit to fixed fesc, which guarantees the completion of the process at z&lt;6.7. The analysis is tested with two limit hypothesis on high-z ionized hydrogen volume fraction, comparing our predictions with observational results

X-Ray Evolution of Active Galactic Nuclei and Hierarchical Galaxy Formation

We have incorporated the description of the X-ray properties of Active Galactic Nuclei (AGNs) into a semi-analytic model of galaxy formation, adopting physically motivated scaling laws for accretion triggered by galaxy encounters. Our model reproduces the level of the cosmic X-ray background at 30 keV; we predict that the largest contribution (around 2/3) comes from sources with intermediate X-ray luminosity 10^{43.5}< L_X/erg/s <10^{44.5}, with 50 % of the total specific intensity produced at z<2. The predicted number density of luminous X-ray AGNs (L_X>10^{44.5} erg/s in the 2-10 keV band) peaks at z around 2 with a decline of around 3 dex to z=0; for the low luminosity sources (10^{43}<L_X/erg/s <10^{44}) it has a broaderand less pronounced maximum around z 1.5. The comparison with the data shows a generally good agreement. The model predictions slightly exceed the observed number of low-luminosity AGNs at z around 1.5, with the discrepancy progressively extending to intermediate-luminosity objects at higher redshifts; we discuss possible origins for the mismatch. Finally, we predict the source counts and the flux distribution at different redshifts in the hard (20-100 keV) X-ray band for the sources contributing to the X- ray background.Comment: 27 pages, accepted for publication in Ap

Unsolved Problems about Supernovae

A number of unsolved problems and open questions about the nature and the properties of supernovae are identified and briefly discussed. Some suggestions and directions toward possible solutions are also considered.Comment: 7 pages, Proceedings of 'Probing Stellar Populations out to the Distant Universe', Cefalu, Italy, Sep 7-19, 2008, AIP Conf. Proc. Serie

The formation and evolution of early-type galaxies : solid results and open questions

The most recent results and some of the open key questions on the evolution of early-type galaxies are reviewed in the general cosmological context of massive galaxy formation.Comment: 8 pages, invited review at the workshop "Probing Stellar Populations out to the Distant Universe", Cefalu` (Italy), September 7 - 19, 200

Spectral libraries and their uncertainties

Libraries of stellar spectra are fundamental tools in the study of stellar populations and in automatic determination of atmospheric parameters for large samples of observed stars. In the context of the present volume, here I give an overview of the current status of stellar spectral libraries from the perspective of stellar population modeling: what we have currently available, how good they are, and where we need further improvement

Advances on GRB as cosmological tools

Several interesting correlations among Gamma Ray Bursts (GRB) prompt and afterglow properties have been found in the recent years. Some of these correlations have been proposed also to standardize GRB energetics to use them as standard candles in constraining the expansion history of the universe up to z>6. However, given the still unexplained nature of most of these correlations, only the less scattered correlations can be used for constraining the cosmological parameters. The updated E_peak-E_gamma correlation is presented. Caveats of alternative methods of standardizing GRB energetics are discussed.Comment: 8 parges, AIP conf. proc. "Probing stellar populations out to the distant universe, Cefalu' 2008" Vol. 1111, pp. 579-58

Do the Early Galaxies observed by JWST disagree with Planck's CMB polarization measurements?

The recent observations from the James Webb Space Telescope have led to a surprising discovery of a significant density of massive galaxies with masses of $M \ge 10^{10.5} M_{\odot}$ at redshifts of approximately $z\sim 10$. This corresponds to a stellar mass density of roughly $\rho_*\sim 10^6 M_{\odot} Mpc^{-3}$. Despite making conservative assumptions regarding galaxy formation, this finding may not be compatible with the standard $\Lambda$CDM cosmology that is favored by observations of CMB Anisotropies from the Planck satellite. In this paper, we confirm the substantial discrepancy with Planck's results within the $\Lambda$CDM framework. Assuming a value of $\epsilon=0.2$ for the efficiency of converting baryons into stars, we indeed find that the $\Lambda$CDM model is excluded at more than $99.7 \%$ confidence level (C.L.). An even more significant exclusion is found for $\epsilon \sim 0.1$, while a better agreement, but still in tension at more than $95 \%$, is obtained for $\epsilon =0.32$. This tension, as already discussed in the literature, could arise either from systematics in the JWST measurements or from new physics. Here, as a last-ditch effort, we point out that disregarding the large angular scale polarization obtained by Planck, which allows for significantly larger values of the matter clustering parameter $\sigma_8$, could lead to better agreement between Planck and JWST within the $\Lambda$CDM framework. Interestingly, the model compatible with Planck temperature-only data and JWST observation also favors a higher Hubble constant $H_0=69.0\pm1.1$ km/s/Mpc at $68\%$ C.L., in better agreement with observations based on SN-Ia luminosity distances.Comment: 5 pages, 2 Figures and a bunch of galaxie