903 research outputs found
Forming Clusters of Galaxies as the Origin of Unidentified GeV Gamma-Ray Sources
Over half of GeV gamma-ray sources observed by the EGRET experiment have not
yet been identified as known astronomical objects. There is an isotropic
component of such unidentified sources, whose number is about 60 in the whole
sky. Here we calculate the expected number of dynamically forming clusters of
galaxies emitting gamma-rays by high energy electrons accelerated in the shock
wave when they form, in the framework of the standard theory of structure
formation. We find that a few tens of such forming clusters should be
detectable by EGRET and hence a considerable fraction of the isotropic
unidentified sources can be accounted for, if about 5% of the shock energy is
going into electron acceleration. We argue that these clusters are very
difficult to detect in x-ray or optical surveys compared with the conventional
clusters, because of their extended angular size of about 1 degree. Hence they
define a new population of ``gamma-ray clusters''. If this hypothesis is true,
the next generation gamma-ray telescopes such as GLAST will detect more than a
few thousands of gamma-ray clusters. It would provide a new tracer of
dynamically evolving structures in the universe, in contrast to the x-ray
clusters as a tracer of hydrodynamically stabilized systems. We also derive the
strength of magnetic field required for the extragalactic gamma-ray background
by structure formation to extend up to 100 GeV as observed, that is about
10^{-5} of the shock-heated baryon energy density.Comment: Accepted by ApJ after minor revisions. Received May 9, Accepted
August 3. 8 pages including 2 figure
Cosmological Implications of the Fundamental Relations of X-ray Clusters
Based on the two-parameter family nature of X-ray clusters of galaxies
obtained in a separate paper, we discuss the formation history of clusters and
cosmological parameters of the universe. Utilizing the spherical collapse model
of cluster formation, and assuming that the cluster X-ray core radius is
proportional to the virial radius at the time of the cluster collapse, the
observed relations among the density, radius, and temperature of clusters imply
that cluster formation occurs in a wide range of redshift. The observed
relations favor the low-density universe. Moreover, we find that the model of
is preferable.Comment: 7 pages, 4 figures. To be published in ApJ Letter
A new measure of using the lensing dispersion in high- type Ia SNe
The gravitational lensing magnification or demagnification due to large-scale
structures induces a scatter in peak magnitudes of high redshift type Ia
supernovae (SNe Ia). The amplitude of the lensing dispersion strongly depends
on that of density fluctuations characterized by the parameter.
Therefore the value of is constrained by measuring the dispersion in
the peak magnitudes. We examine how well SN Ia data will provide a constraint
on the value of using a likelihood analysis method. It is found that
the number and quality of SN Ia data needed for placing a useful constraint on
is attainable with Next Generation Space Telescope.Comment: 9 pages, 3 figures. Accepted for publication in The Astrophysical
Journa
Heating of the IGM
Using the cosmic virial theorem, Press-Schechter analysis and numerical
simulations, we compute the expected X-ray background (XRB) from the diffuse
IGM with the clumping factor expected from gravitational shock heating. The
predicted fluxes and temperatures are excluded from the observed XRB. The
predicted clumping can be reduced by entropy injection. The required energy is
computed from the two-point correlation function, as well as from
Press-Schechter formalisms. The minimal energy injection of 1 keV/nucleon
excludes radiative or gravitational heating as a primary energy source. We
argue that the intergalactic medium (IGM) must have been heated through violent
processes such as massive supernova bursts. If the heating proceeded through
supernova explosions, it likely proceeded in bursts which may be observable in
high redshift supernova searches. Within our model we reproduce the observed
cluster luminosity-temperature relation with energy injection of 1 keV/nucleon
if this injection is assumed to be uncorrelated with the local density. These
parameters predict that the diffuse IGM soft XRB has a temperature of ~1 keV
with a flux near 10 keV/cm^2 s str keV, which may be detectable in the near
future.Comment: to appear in ApJ Lett., 11 pages incl 1 figur
Anisotropic Galactic Outflows and Enrichment of the Intergalactic Medium. I: Monte Carlo Simulations
We have developed an analytical model to describe the evolution of
anisotropic galactic outflows. With it, we investigate the impact of varying
opening angle on galaxy formation and the evolution of the IGM. We have
implemented this model in a Monte Carlo algorithm to simulate galaxy formation
and outflows in a cosmological context. Using this algorithm, we have simulated
the evolution of a comoving volume of size [12h^(-1)Mpc]^3 in the LCDM
universe. Starting from a Gaussian density field at redshift z=24, we follow
the formation of ~20,000 galaxies, and simulate the galactic outflows produced
by these galaxies. When these outflows collide with density peaks, ram pressure
stripping of the gas inside the peak may result. This occurs in around half the
cases and prevents the formation of galaxies. Anisotropic outflows follow the
path of least resistance, and thus travel preferentially into low-density
regions, away from cosmological structures (filaments and pancakes) where
galaxies form. As a result, the number of collisions is reduced, leading to the
formation of a larger number of galaxies. Anisotropic outflows can
significantly enrich low-density systems with metals. Conversely, the
cross-pollution in metals of objects located in a common cosmological
structure, like a filament, is significantly reduced. Highly anisotropic
outflows can travel across cosmological voids and deposit metals in other,
unrelated cosmological structures.Comment: 32 pages, 9 figures (2 color). Revised version accepted in Ap
Normalizing the Temperature Function of Clusters of Galaxies
We re-examine the constraints which can be robustly obtained from the
observed temperature function of X-ray cluster of galaxies. The cluster mass
function has been thoroughly studied in simulations and analytically, but a
direct simulation of the temperature function is presented here for the first
time. Adaptive hydrodynamic simulations using the cosmological Moving Mesh
Hydro code of Pen (1997a) are used to calibrate the temperature function for
different popular cosmologies. Applying the new normalizations to the
present-day cluster abundances, we find for a hyperbolic universe, and for a spatially flat universe with a cosmological constant.
The simulations followed the gravitational shock heating of the gas and dark
matter, and used a crude model for potential energy injection by supernova
heating. The error bars are dominated by uncertainties in the heating/cooling
models. We present fitting formulae for the mass-temperature conversions and
cluster abundances based on these simulations.Comment: 20 pages incl 5 figures, final version for ApJ, corrected open
universe \gamma relation, results unchange
Weak Lensing as a Calibrator of the Cluster Mass-Temperature Relation
The abundance of clusters at the present epoch and weak gravitational lensing
shear both constrain roughly the same combination of the power spectrum
normalization sigma_8 and matter energy density Omega_M. The cluster constraint
further depends on the normalization of the mass-temperature relation.
Therefore, combining the weak lensing and cluster abundance data can be used to
accurately calibrate the mass-temperature relation. We discuss this approach
and illustrate it using data from recent surveys.Comment: Matches the version in ApJL. Equation 4 corrected. Improvements in
the analysis move the cluster contours in Fig1 slightly upwards. No changes
in the conclusion
Mass-Temperature Relation of Galaxy Clusters: A Theoretical Study
Combining conservation of energy throughout nearly-spherical collapse of
galaxy clusters with the virial theorem, we derive the mass-temperature
relation for X-ray clusters of galaxies . The normalization factor
and the scatter of the relation are determined from first principles with
the additional assumption of initial Gaussian random field. We are also able to
reproduce the recently observed break in the M-T relation at T \sim 3 \keV,
based on the scatter in the underlying density field for a low density
CDM cosmology. Finally, by combining observational data of high
redshift clusters with our theoretical formalism, we find a semi-empirical
temperature-mass relation which is expected to hold at redshifts up to unity
with less than 20% error.Comment: 43 pages, 13 figures, One figure is added and minor changes are made.
Accepted for Publication in Ap
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