150 research outputs found
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 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: .Comment: Accepted for publication in MNRAS (2 figures added
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, and the 5 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, V, B, U), using simulations to account for
incompleteness and spurious sources contamination. Along with photometry in all
bands, we determined on the I-selected catalogue (I)
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
Gamma-ray burst optical light-curve zoo: comparison with X-ray observations
We present a comprehensive analysis of the optical and X-ray light curves
(LCs) and spectral energy distributions (SEDs) of a large sample of gamma-ray
burst (GRB) afterglows to investigate the relationship between the optical and
X-ray emission after the prompt phase. We collected the optical data from the
literature and determined the shapes of the optical LCs. Then, using previously
presented X-ray data we modeled the optical/X-ray SEDs. We studied the SED
parameter distributions and compared the optical and X-ray LC slopes and
shapes. The optical and X-ray spectra become softer as a function of time while
the gas-to-dust ratios of GRBs are higher than the values calculated for the
Milky Way and the Large and Magellanic Clouds. For 20% of the GRBs the
difference between the optical and X-ray slopes is consistent with 0 or 1=4
within the uncertainties (we did it not consider the steep decay phase), while
in the remaining 80% the optical and X-ray afterglows show significantly
different temporal behaviors. Interestingly, we find an indication that the
onset of the forward shock in the optical LCs (initial peaks or shallow phases)
could be linked to the presence of the X-ray flares. Indeed, when X-ray flares
are present during the steep decay, the optical LC initial peak or end plateau
occurs during the steep decay; if instead the X-ray flares are absent or occur
during the plateau, the optical initial peak or end plateau takes place during
the X-ray plateau. The forward-shock model cannot explain all features of the
optical (e.g. bumps, late re-brightenings) and X-ray (e.g. flares, plateaus)
LCs. However, the synchrotron model is a viable mechanism for GRBs at late
times. In particular, we found a relationship between the presence of the X-ray
flares and the shape of the optical LC that indicates a link between the prompt
emission and the optical afterglow.Comment: 55 pages, 37 figures, accepted for publication in A&A (this version
includes changes made at Proofs stage
- …