38 research outputs found
The Effect of Cooling on the Density Profile of Hot Gas in Clusters of Galaxies: Is Additional Physics Needed?
We use high-resolution hydrodynamic simulations to investigate the density
profile of hot gas in clusters of galaxies, adopting a variant of cold dark
matter cosmologies and employing a cosmological N-body/smoothed particle
hydrodynamics code to follow the evolution of dark matter and gas. In addition
to gravitational interactions, gas pressure, and shock heating, we include
bremsstrahlung cooling in the computation. Dynamical time, two-body relaxation
time, and cooling time in the simulations are examined to demonstrate that the
results are free from artificial relaxation effects and that the time step is
short enough to accurately follow the evolution of the system. In the
simulation with nominal resolution of 66h^{-1} kpc the computed cluster appears
normal, but in a higher (by a factor 2) resolution run, cooling is so efficient
that the final gas density profile shows a steep rise toward the cluster center
that is not observed in real clusters. Also, the X-ray luminosity of
7\times10^{45} ergs s^{-1} far exceeds that for any cluster of the computed
temperature. The most reasonable explanation for this discrepancy is that there
are some physical processes still missing in the simulations that actually
mitigate the cooling effect and play a crucial role in the thermal and
dynamical evolution of the gas near the center. Among the promising candidate
processes are heat conduction and heat input from supernovae. We discuss the
extent to which these processes can alter the evolution of gas.Comment: 19 pages, 5 postscript figures included; uses aaspp4.sty (AASTeX
v4.0); title changed; final version published in The Astrophysical Journa
Probability Distribution Function of Cosmological Density Fluctuations from Gaussian Initial Condition: Comparison of One- and Two-point Log-normal Model Predictions with N-body Simulations
We quantitatively study the probability distribution function (PDF) of
cosmological nonlinear density fluctuations from N-body simulations with
Gaussian initial condition. In particular, we examine the validity and
limitations of one-point and two-point log-normal PDF models against those
directly estimated from the simulations. We find that the one-point log-normal
PDF describes very accurately the cosmological density distribution even in the
nonlinear regime (the rms variance \sigma_{nl} \simlt 4 and the over-density
\delta \simlt 100). Furthermore the two-point log-normal PDFs are also in good
agreement with the simulation data from linear to fairly nonlinear regime,
while slightly deviate from them for \delta \simlt -0.5. Thus the log-normal
PDF can be used as a useful empirical model for the cosmological density
fluctuations. While this conclusion is fairly insensitive to the shape of the
underlying power spectrum of density fluctuations P(k), models with substantial
power on large scales, i.e., n\equiv d\ln P(k)/d \ln k \simlt -1, are better
described by the log-normal PDF. On the other hand, we note that the one-to-one
mapping of the initial and the evolved density fields consistent with the
log-normal model does not approximate the broad distribution of their mutual
correlation even on average. Thus the origin of the phenomenological log-normal
PDF approximation still remains to be understood.Comment: 25 pages, 8 figures, Accepted for publication in Ap
PACSWIN: A new international ocean climate program in the Indonesian seas and adjacent regions
Measuring our universe from galaxy redshift surveys
Galaxy redshift surveys have achieved significant progress over the last
couple of decades. Those surveys tell us in the most straightforward way what
our local universe looks like. While the galaxy distribution traces the bright
side of the universe, detailed quantitative analyses of the data have even
revealed the dark side of the universe dominated by non-baryonic dark matter as
well as more mysterious dark energy (or Einstein's cosmological constant). We
describe several methodologies of using galaxy redshift surveys as cosmological
probes, and then summarize the recent results from the existing surveys.
Finally we present our views on the future of redshift surveys in the era of
Precision Cosmology.Comment: 82 pages, 31 figures, invited review article published in Living
Reviews in Relativity, http://www.livingreviews.org/lrr-2004-
Comparison of the Sachs-Wolfe Effect for Gaussian and Non-Gaussian Fluctuations
A consequence of non-Gaussian perturbations on the Sachs-Wolfe effect is
studied. For a particular power spectrum, predicted Sachs-Wolfe effects are
calculated for two cases: Gaussian (random phase) configuration, and a specific
kind of non-Gaussian configuration. We obtain a result that the Sachs-Wolfe
effect for the latter case is smaller when each temperature fluctuation is
properly normalized with respect to the corresponding mass fluctuation . The physical explanation and the generality of the result are
discussed.Comment: 16 page
CMB B-polarization to map the Large-scale Structures of the Universe
We explore the possibility of using the B-type polarization of the CMB to map
the large-scale structures of the Universe taking advantage of the lens effects
on the CMB polarization. The functional relation between the B component with
the primordial CMB polarization and the line-of-sight mass distribution is
explicited. Noting that a sizeable fraction (at least 40%) of the dark halo
population which is responsible of this effect can also be detected in galaxy
weak lensing survey, we present statistical quantities that should exhibit a
strong sensitivity to this overlapping. We stress that it would be a sound test
of the gravitational instability picture, independent on many systematic
effects that may hamper lensing detection in CMB or galaxy survey alone.
Moreover we estimate the intrinsic cosmic variance of the amplitude of this
effect to be less than 8% for a 100, deg^2 survey with a 10' CMB beam. Its
measurement would then provide us with an original mean for constraining the
cosmological parameters, more particularly, as it turns out, the cosmological
constant Lambda.Comment: Latex2e with REVTEX ; 14 pages, 8 figure
Systematic Errors in the Hubble Constant Based on Measurement of the Sunyaev-Zeldovich Effect
Values of the Hubble constant reported to date which are based on measurement
of the Sunyaev-Zeldovich (SZ) effect in clusters of galaxies are systematically
lower than those derived by other methods (e.g., Cepheid variable stars, or the
Tully-Fisher relation). We investigate the possibility that systematic errors
may be introduced into the analysis by the generally adopted assumptions that
observed clusters are in hydrostatic equilibrium, are spherically symmetric,
and are isothermal. We construct self-consistent theoretical models of merging
clusters of galaxies using hydrodynamical/N-body simulations. We then compute
the magnitude of Ho derived from the SZ effect at different times and at
different projection angles both from first principles, and by applying each of
the standard assumptions used in the interpretation of observations. Our
results indicate that the assumption of isothermality in the evolving clusters
can result in Ho being underestimated by 10-30% depending on both epoch and
projection angle. Moreover, use of the projected, emission-weighted temperature
profile under the assumption of spherical symmetry does not significantly
improve the situation except in the case of more extreme mergers (i.e., those
involving relatively gas-rich subclusters).Comment: 31 pages, Latex, 2 tables, 10 postscript figures, Accepted for
publication in ApJ, scheduled for June 20, 199
The effect of non--gravitational gas heating in groups and clusters of galaxies
We present a set of gas-dynamical simulations of galaxy groups and clusters
aimed at exploring the effect of non-gravitational heating. We use GASOLINE, a
parallel Tree+SPH code, to simulate the formation of four cosmic halos with
temperature 0.5<T<8 keV. Non-gravitational heating is implemented in two
different ways: (1) by imposing a minimum entropy floor at a given redshift,
1<z<5; (2) by gradually heating gas, proportionally to the SN rate expected
from semi-analytical modeling of galaxy formation. Our main results are the
following. (a) An extra heating energy of about 1 keV per gas particle is
required to reproduce the observed Lx-T relation, independent of whether it is
provided so as to create an entropy floor of 50-100 keV cm^2, or is modulated
in redshift; our SN feedback recipe provides only 1/3 keV/part. (b) The M-T
relation is almost unaffected by non-gravitational heating and follows the M
T^{3/2} scaling, with a normalization ~40% higher than observed, independent of
the heating scheme. The inclusion of cooling in a run of a small group has the
effects of increasing T_ew by ~30%, possibly reconciling simulated and observed
M-T relations, and of decreasing Lx by ~40%. In spite of the inclusion of SN
feedback energy, almost 40% of the gas becomes cold, in excess of current
observational estimates. (abridged)Comment: 18 pages, 15 figures, to appear in MNRAS. Version with high
resolution images available at
http://www.daut.univ.trieste.it/borgani/LT/lt_1.ps.g
The Formation of the First Stars in the Universe
In this review, I survey our current understanding of how the very first
stars in the universe formed, with a focus on three main areas of interest: the
formation of the first protogalaxies and the cooling of gas within them, the
nature and extent of fragmentation within the cool gas, and the physics -- in
particular the interplay between protostellar accretion and protostellar
feedback -- that serves to determine the final stellar mass.
In each of these areas, I have attempted to show how our thinking has
developed over recent years, aided in large part by the increasing ease with
which we can now perform detailed numerical simulations of primordial star
formation. I have also tried to indicate the areas where our understanding
remains incomplete, and to identify some of the most important unsolved
problems.Comment: 74 pages, 4 figures. Accepted for publication in Space Science
Review