The Halo Density Profiles with Non-Standard N-body Simulations

We propose a new numerical procedure to simulate a single dark halo of any size and mass in a hierarchical framework coupling the extended Press-Schechter formalism (EPSF) to N-body simulations. The procedure consists of assigning cosmological initial conditions to the particles of a single halo with a EPSF technique and following only the dynamical evolution using a serial N-body code. The computational box is fixed with a side of $0.5 h^{-1}$ Mpc. This allows to simulate galaxy cluster halos using appropriate scaling relations, to ensure savings in computing time and code speed. The code can describe the properties of halos composed of collisionless or collisional dark matter. For collisionless Cold Dark Matter (CDM) particles the NFW profile is reproduced for galactic halos as well as galaxy cluster halos. Using this numerical technique we study some characteristics of halos assumed to be isolated or placed in a cosmological context in presence of weak self-interacting dark matter: the soft core formation and the core collapse. The self-interacting dark matter cross section per unit mass is assumed to be inversely proportional to the particle collision velocity: $\sigma/m_{x} \propto 1/v$.Comment: Accepted for publication in MNRAS (2 figures added

The Gamma Ray Burst Luminosity Function in the Light of the Swift 2-year Data

We compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by the BATSE satellite in three different scenarios: i) GRBs follow the cosmic star formation and their LF is constant in time; ii) GRBs follow the cosmic star formation but the LF varies with redshift; iii) GRBs form preferentially in low-metallicity environments. We find that the differential peak flux number counts obtained by BATSE and by Swift can be reproduced using the same LF and GRB formation rate, indicating that the two satellites are observing the same GRB population. We then check the resulting redshift distributions in the light of Swift 2-year data, focusing in particular on the relatively large sample of GRBs detected at z>2.5. We show that models in which GRBs trace the cosmic star formation and are described by a constant LF are ruled out by the number of high-z Swift detections. This conclusion does not depend on the redshift distribution of bursts that lack of optical identification, nor on the existence of a decline in star formation rate at z>2, nor on the adopted faint-end of the GRB LF. Swift observations can be explained by assuming that the LF varies with redshift and/or that GRB formation is limited to low-metallicity environments.Comment: 7 pages, 3 figures, ApJ Letter in pres

A Catalogue of Galaxies in the HDF-South: Photometry and Structural Parameters

We describe the construction of a catalogue of galaxies in the optical field of the Hubble Deep Field South. The HDF-S observations produced WFPC2 images in U, B, V, and I, the version 1 data have been made public on 23 November 1999. The effective field of view is 4.38 arcmin$^2$, and the 5$\sigma$ limiting magnitudes (in a FWHM aperture) are 28.87, 29.71, 30.19, 29.58 in the U, B, V and I bands, respectively. We created a catalogue for each pass-band (I$_{814}$, V$_{606}$, B$_{450}$, U$_{300}$), using simulations to account for incompleteness and spurious sources contamination. Along with photometry in all bands, we determined on the I$_{814}$-selected catalogue (I$_{814}<26$) structural parameters, such as a metric apparent size, derived from the petrosian radius, an asymmetry index, light concentration indexes and the mean surface brightness within the petrosian radius.Comment: 10 pages, 11 figures. Accepted for publication in A&ASS. The catalog is available in the source and at http://www.merate.mi.astro.it/~saracco/science.htm