255 research outputs found

### Cosmic-Ray Proton to Electron Ratios

A basic quantity in the characterization of relativistic particles is the
proton-to-electron (p/e) energy density ratio. We derive a simple approximate
expression suitable to estimate this quantity, U_p/U_e = (m_p/m_e)^(3-q)/2,
valid when a nonthermal `gas' of these particles is electrically neutral and
the particles' power-law spectral indices are equal -- e.g., at injection. This
relation partners the well-known p/e number density ratio at 1 GeV, i.e.
N_p/N_e = (m_p/m_e)^{(q-1)/2}.Comment: 4 pages; to be published in Proc. of MGM13 (13th Marcel Grossmann
Meeting -- Stockholm July 1-7, 2012

### Impact of a non-Gaussian density field on Sunyaev-Zeldovich observables

The main statistical properties of the Sunyaev-Zeldovich (S-Z) effect - the
power spectrum, cluster number counts, and angular correlation function - are
calculated and compared within the framework of two density fields which differ
in their predictions of the cluster mass function at high redshifts. We do so
for the usual Press and Schechter mass function, which is derived on the basis
of a Gaussian density fluctuation field, and for a mass function based on a
chi^2 distributed density field. These three S-Z observables are found to be
very significantly dependent on the choice of the mass function. The different
predictions of the Gaussian and non-Gaussian density fields are probed in
detail by investigating the behaviour of the three S-Z observables in terms of
cluster mass and redshift. The formation time distribution of clusters is also
demonstrated to be sensitive to the underlying mass function. A
semi-quantitative assessment is given of its impact on the concentration
parameter and the temperature of intracluster gas.Comment: 17 pages, 11 figures, accepted for publication in MNRA

### The Effects of Relativistic Corrections on Cosmological Parameter Estimations from SZE Cluster Surveys

Sunyaev-Zel'dovich Effect (SZE) cluster surveys are anticipated to yield
tight constraints on cosmological parameters such as the equation of state of
dark energy. In this paper, we study the impact of relativistic corrections of
the thermal SZE on the cluster number counts expected from a cosmological model
and thus, assuming that other cosmological parameters are known to high
accuracies, on the determination of the $w$ parameter and $\sigma_8$ from a SZE
cluster survey, where $w=p/\rho$ with $p$ the pressure and $\rho$ the density
of dark energy, and $\sigma_8$ is the rms of the extrapolated linear density
fluctuation smoothed over $8\hbox{Mpc}h^{-1}$. For the purpose of illustrating
the effects of relativistic corrections, our analyses mainly focus on $\nu=353
\hbox{GHz}$ and $S_{lim}=30\hbox{mJy}$, where $\nu$ and $S_{lim}$ are the
observing frequency and the flux limit of a survey, respectively. These
observing parameters are relevant to the {\it Planck} survey. It is found that
from two measurable quantities, the total number of SZE clusters and the number
of clusters with redshift $z\ge 0.5$, $\sigma_8$ and $w$ can be determined to a
level of $\pm 1%$ and $\pm 8%$, respectively, with $1\sigma$ uncertainties from
a survey of $10000\hbox{deg}^2$. Relativistic effects are important in
determining the central values of $\sigma_8$ and $w$. If we choose the two
quantities calculated relativistically from the flat cosmological model with
$\sigma_8=0.8284$ and $w=-0.75$ as input, the derived $\sigma_8$ and $w$ would
be 0.819 and -0.81, respectively, if relativistic effects are wrongly
neglected. The location of the resulting $\sigma_8$ and $w$ in the $\sigma_8-w$
plane is outside the $3\sigma$ region around the real central $\sigma_8$ and
$w$.Comment: ApJ in pres

### On the origin of extinction in the Coma cluster of galaxies

Visual extinction of distant clusters seen through the Coma cluster seem to suggest that dust may be present in the hot x ray emitting intracluster gas. However, the Infrared Astronomy Satellite (IRAS) failed to detect any infrared emission from the cluster at the level expected from the extinction measurements. Researchers carried out a detailed analysis of the properties of intracluster dust in the context of a model which includes continuous injection of dust by the cluster galaxies, grain destruction by sputtering, and transient grain heating by the hot plasma. Computed infrared fluxes are in agreement with the upper limit obtained from the IRAS. The calculations, and the constraint implied by the IRAS observations, suggest that the intracluster dust must be significantly depleted compared to interstellar abundances. Researchers discuss possible explanations for the discrepancy between the observed visual extinction and the IRAS upper limit

### The Sunyaev-Zeldovich Effect and Its Cosmological Significance

Comptonization of the cosmic microwave background (CMB) radiation by hot gas
in clusters of galaxies - the Sunyaev-Zeldovich (S-Z) effect - is of great
astrophysical and cosmological significance. In recent years observations of
the effect have improved tremendously; high signal-to-noise images of the
effect (at low microwave frequencies) can now be obtained by ground-based
interferometric arrays. In the near future, high frequency measurements of the
effect will be made with bolomateric arrays during long duration balloon
flights. Towards the end of the decade the PLANCK satellite will extensive S-Z
surveys over a wide frequency range. Along with the improved observational
capabilities, the theoretical description of the effect and its more precise
use as a probe have been considerably advanced. I review the current status of
theoretical and observational work on the effect, and the main results from its
use as a cosmological probe.Comment: Invited review; in proceedings of the Erice NATO/ASI `Astrophysical
Sources of High Energy Particles and Radiation'; 11 pages, 3 figure

### Determination of cosmological parameters: an introduction for non-specialists

I start by defining the cosmological parameters $H_0, \Omega_m$ and
$\Omega_\Lambda$. Then I show how the age of the universe depends on them,
followed by the evolution of the scale parameter of the universe for various
values of the density parameters. Then I define strategies for measuring them,
and show the results for the recent determination of these parameters from
measurements on supernovas of type 1a. Implications for particle physics is
briefly discussed at the end.Comment: 12 pages, Latex with epsf.sty. Invited talk at the ``Discussion
meeting on Recent Developments in Neutrino Physics'', held at the Physical
Research Laboratory, Ahmedabad, February 2--4, 199

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