919 research outputs found
Continuous matter creation and the acceleration of the universe: the growth of density fluctuations
Cosmologies including continuous matter creation are able to reproduce the
main properties of the standard CDM model, in particular in cases
where the particle and entropy production rates are equal. These specific
models, characterized by a mass density equal to the critical value, behave
like the standard CDM model at early times whereas their late
evolution is similar to the steady-state cosmology. The maximum amplitude of
density fluctuations in these models depends on the adopted creation rate,
related here to the parameter and this limitation could be a
difficulty for the formation of galaxies and large-scale structure in this
class of universe. Additional problems are related with predictions either of
the random peculiar velocities of galaxies or the present density of massive
clusters of galaxies, both being largely overestimated with respect to
observational data.Comment: 11 pages, 2 figures, accepted for publication in General Relativity
and Gravitatio
The Evolution of the Baryon Distribution in the Universe from Cosmological Simulations
The evolution of the baryon distribution in different phases, derived from
cosmological simulations, are here reported. These computations indicate that
presently most of baryons are in a warm-hot intergalactic (WHIM) medium (about
43%) while at z = 2.5 most of baryons constitute the diffuse medium (about
74%). Stars and the cold gas in galaxies represent only 14% of the baryons at z
= 0. For z < 4 about a half of the metals are locked into stars while the
fraction present in the WHIM and in the diffuse medium increases with a
decreasing redshift. In the redshift range 0 < z < 2.5, the amount of metals in
the WHIM increases from 4% to 22% while in the diffuse medium it increases from
0.6% to 4%. This enrichment process is due essentially to a turbulent diffusion
mechanism associated to mass motions driven by supernova explosions. At z = 0,
simulated blue (late type) galaxies show a correlation of the oxygen abundance
present in the cold gas with the luminosity of the considered galaxy that
agrees quite well with data derived from HII regions.Comment: 12 pages, 6 figures, to be published in IJMP
Tidal Disruption Flares: The Accretion Disk Phase
The evolution of an accretion disk, formed as a consequence of the disruption
of a star by a black hole, is followed by solving numerically the hydrodynamic
equations. The present investigation aims to study the dependence of resulting
light curves on dynamical and physical properties of such a transient disk
during its existence. One of main results derived from our simulations is that
black body fits of X-ray data tend to overestimate the true mean disk
temperature. The temperature derived from black body fits should be identified
with the color X-ray temperature rather than the average value derived from the
true temperature distribution along the disk. The time interval between the
beginning of the circularization of the bound debris and the beginning of the
accretion process by the black hole is determined by the viscous timescale,
which fixes also the raising part of the resulting light curve. The luminosity
peak coincides with the beginning of matter accretion by the black hole and the
late evolution of the light curve depends on the evolution of the debris
fallback rate. Peak bolometric luminosities are in the range 10^45-10^46 erg
s^-1 whereas peak luminosities in soft X-rays (0.2-2.0 keV) are typically one
order of magnitude lower. The timescale derived from our preferred models for
the flare luminosity to decay by two orders of magnitude is about 3-4 years.
Predicted soft X-ray light curves were fitted to data on galaxies in which a
variable X-ray emission, related to tidal events, was detected.Comment: 14 pages, 11 figures, Accepted for publication in Ap
Gravitation Wave Emission from Radio Pulsars Revisited
We report a new pulsar population synthesis based on Monte Carlo techniques,
aiming to estimate the contribution of galactic radio pulsars to the continuous
gravitational wave emission. Assuming that the rotation periods of pulsars at
birth have a Gaussian distribution, we find that the average initial period is
290 ms. The number of objects with periods equal to or less than 0.4 s, and
therefore capable of being detected by an interferometric gravitational antenna
like VIRGO, is of the order of 5100-7800. With integration times lasting
between 2 and 3 yr, our simulations suggest that about two detections should be
possible, if the mean equatorial ellipticity of the pulsars is
=10. A mean ellipticity an order of magnitude higher increases the
expected number of detections to 12-18, whereas for , no
detections are expectedComment: accepted for publication in A&A, 9 pages, 8 figure
Evolution of the Phase-Space Density of Dark Matter Halos and Mixing Effects in Merger Events
Cosmological N-body simulations were performed to study the evolution of the
phase-space density Q = rho/sigma^3 of dark matter halos. No significant
differences in the scale relations Q ~ sigma^(-2.1) or Q ~ M^(-0.82) are seen
for "cold" or "warm" dark matter models. The follow up of individual halos from
z = 10 up to the present time indicate the existence of two main evolutionary
phases: an early and fast one (10 > z > 6.5), in which Q decreases on the
average by a factor of 40 as a consequence of the randomization of bulk motions
and a late and long one (6.5 > z > 0), in which Q decreases by a factor of 20
because of mixing induced by merger events. The study of these halos has also
evidenced that rapid and positive variations of the velocity dispersion,
induced by merger episods, are related to a fast decrease of the phase density
Q.Comment: 6 pages, accepted by MNRA
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