arXiv subjects

S. Colafrancesco

Publications and source records attributed to S. Colafrancesco.

241 records · Page 14Linked to original sources

arXiv1998

Clusters of Galaxies and the Diffuse Gamma Ray Background

We discuss the diffuse emission of gamma rays and neutrinos from galaxy clusters in the viable models for structure formation in the universe. We use a self-consistent picture for cluster formation and evolution starting from a primordial density perturbation spectrum, and a realistic modelling for the distribution of the intergalactic medium which is abundantly present within galaxy clusters. We find that an evolving population of clusters can produce a fraction $\sim 0.5 ÷2 %$ of the diffuse gamma-ray background (DGRB) observed by EGRET. This result is robust and is weakly dependent on the cosmological scenario and on the degree of evolution of the inter galactic medium (IGM) in distant clusters, because the bulk of the sources contributing to the DGRB is located at redshifts $z \simlt 0.2$. We also found a correlation between the non-thermal, gamma-ray and the thermal X-ray emissions from these structures. Using this result, we derived a list of gamma-ray clusters observable with the next generation $γ$-ray detectors. Finally, we briefly discuss the possible relevance of galaxy clusters for neutrino astronomy and for very high energy particle astronomy.

astro-ph↗

arXiv1997

A possible theoretical explanation of metallicity gradients in elliptical galaxies

Models of chemical evolution of elliptical galaxies taking into account different escape velocities at different galactocentric radii are presented. As a consequence of this, the chemical evolution develops differently in different galactic regions; in particular, we find that the galactic wind, powered by supernovae (of type II and I) starts, under suitable conditions, in the outer regions and successively develops in the central ones. The rate of star formation (SFR) is assumed to stop after the onset of the galactic wind in each region. The main result found in the present work is that this mechanism is able to reproduce metallicity gradients, namely the gradients in the $Mg_2$ index, in good agreement with observational data. We also find that in order to honor the constant [Mg/Fe] ratio with galactocentric distance, as inferred from metallicity indices, a variable initial mass function as a function of galactocentric distance is required. This is only a suggestion since trends on abundances inferred just from metallicity indices are still uncertain.

astro-ph↗

arXiv1997

Is the Cluster Temperature Function a Reliable Test for $Ω_{0}$?

We discuss the evolution of the cluster temperature function (TF) in different scenarios for structure formation. We use the commonly adopted procedure of fitting the model parameters to the local TF data, finding the best fit values and, most of all, the associated statistical errors. These errors yield an uncertainty in the prediction of the TF evolution. We conclude that, at the moment, observations of cluster temperatures at z<=0.5 could provide only a weak test for $Ω_0$.

astro-ph↗

arXiv1997

Intracluster Comptonization of the CMB: Mean Spectral Distrortion and Cluster Number Counts

The mean sky-averaged Comptonization parameter, y, describing the scattering of the CMB by hot gas in clusters of galaxies is calculated in an array of flat and open cosmological and dark matter models. The models are globally normalized to fit cluster X-ray data, and intracluster gas is assumed to have evolved in a manner consistent with current observations. We predict values of y lower than the COBE/FIRAS upper limit. The corresponding values of the overall optical thickness to Compton scattering are < 10^{-4} for relevant parameter values. Of more practical importance are number counts of clusters across which a net flux (with respect to the CMB) higher than some limiting value can be detected. Such number counts are specifically predicted for the COBRAS/SAMBA and BOOMERANG missions.

astro-ph↗

arXiv1995

Velocity correlation tensor and substructure's statistics around density peaks

We derive the conditional probability distribution of the peculiar velocity around a peak of given overdensity in a Gaussian density field. This distribution characterizes the shear field around local, isolated density peaks. We use the moments of the velocity distribution to study the spatial distribution of the escape velocity from protostructures forming in a standard, flat, scale--invariant CDM model. In the case of high ($ν\sim 2$) peaks with filtering scales $R_f \sim 2 h^{-1}~\mbox{Mpc}$, we find that at distances between $2 ÷4~{\rm \mbox{Mpc}~h^{-1}}$ (or equivalently $\approx 1 ÷2 ~R_f$) a sensitive fraction of the matter enclosed within an extrapolated (up to the present epoch) volume containing a total gravitating mass $M \approx 2.725\times10^{15} h^{-1} M_{\odot}$ (of the same order as that derived from gravitational lensing observations, see Tyson and Fisher~\cite{tf95}) is {\em gravitationally unbound} to the peak. The fraction of unbound matter varies between approximately between $0.05-0.2$ depending on the central height $ν$ of the density peak, on the filtering radius and on the adopted normalization for the fluctuation spectrum. We show how these results support our previous conclusions regarding the delay of the collapse induced by the presence of small-scale substructure in clusters of galaxies formed in standard CDM cosmological models.

astro-ph↗

arXiv1995

Observational constraints on blue primordial spectra

Recently, there has been growing interest on primordial ``blue" ($n>1$) perturbation spectra, motivated both by a composite set of observational data on large scales and from the point of view of theoretical model building. After reviewing the theoretical (inflationary) motivations for these blue spectra, we consider various observational constraints, within both Cold Dark Matter and Mixed Dark Matter scenarios. In particular, using linear theory, we discuss large--scale bulk flows and the X--ray cluster abundance. We also perform various N--body simulations of these models to study the clustering properties of the matter distribution and the peculiar velocity field.

astro-ph↗

arXiv1994

On the Dynamical Origin of Bias in Clusters of Galaxies

We study the effect of the dynamical friction induced by the presence of substructure on the statistics of the collapse of density peaks. Applying the results of a former paper we show that within high density environments, like rich clusters of galaxies, the collapse of smaller peaks is strongly delayed until very late epochs. A bias of dynamical nature thus naturally arises because high density peaks preferentially collapse For a standard CDM model we find that this dynamical bias can account for a substantial part of the total bias required by observations on cluster scales.

astro-ph↗