1,153 research outputs found
Background radiation from sterile neutrino decay and reionization
Sterile neutrinos are one of the most promising Warm Dark Matter candidates.
By considering their radiative- and pion-decay channels, we derive the allowed
contribution of sterile neutrinos to the X-ray, optical and near-infrared
cosmic backgrounds. The X-ray background puts a strong constraint on the mass
of radiatively decaying neutrinos (m <= 14 keV), whereas the allowed mass range
for pion-decay neutrinos (for a particle lifetime > 4 X 10^17 s) is 150 <=
m/MeV <= 500. Taking into account these constraints, we find that sterile
neutrinos do not significantly contribute to the optical and near-infrared
background. We further consider the impact of sterile neutrinos on
reionization. We find that the Thomson optical depth due to sterile neutrinos
is tau_e = (0.4-3) X 10^-2 in the case of radiative decays, and it is ~10^-3
for the pion-decay channel. We conclude that these particles must have played
only a minor role in cosmic reionization history.Comment: 13 pages, 9 figures, replaced with revised version, accepted for
publication in MNRA
Dark matter implications of the WMAP-Planck Haze
Gamma rays and microwave observations of the Galactic Center and surrounding
areas indicate the presence of anomalous emission, whose origin remains
ambiguous. The possibility of dark matter (DM) annihilation explaining both
signals through prompt emission at gamma-rays and secondary emission at
microwave frequencies from interactions of high-energy electrons produced in
annihilation with the Galactic magnetic fields has attracted much interest in
recent years. We investigate the DM interpretation of the Galactic Center
gamma-ray excess by searching for the associated synchrotron in the WMAP-Planck
data. Considering various magnetic field and cosmic-ray propagation models, we
predict the synchrotron emission due to DM annihilation in our Galaxy, and
compare it with the WMAP-Planck data at 23-70GHz. In addition to standard
microwave foregrounds, we separately model the microwave counterpart to the
Fermi Bubbles and the signal due to DM, and use component separation techniques
to extract the signal associated with each template from the total emission. We
confirm the presence of the Haze at the level of 7% of the total sky intensity
at 23GHz in our chosen region of interest, with a harder spectrum than the synchrotron from regular cosmic-ray electrons. The data do
not show a strong preference towards fitting the Haze by either the Bubbles or
DM emission only. Inclusion of both components provides a better fit with a DM
contribution to the Haze emission of 20% at 23GHz, however, due to significant
uncertainties in foreground modeling, we do not consider this a clear detection
of a DM signal. We set robust upper limits on the annihilation cross section by
ignoring foregrounds, and also report best-fit DM annihilation parameters
obtained from a complete template analysis. We conclude that the WMAP-Planck
data are consistent with a DM interpretation of the gamma-ray excess.Comment: 34 pages, 9 figure
A new cosmic microwave background constraint to primordial gravitational waves
Primordial gravitational waves (GWs) with frequencies > 10^{-15} Hz
contribute to the radiation density of the Universe at the time of decoupling
of the cosmic microwave background (CMB). The effects of this GW background on
the CMB and matter power spectra are identical to those due to massless
neutrinos, unless the initial density-perturbation amplitude for the
gravitational-wave gas is non-adiabatic, as may occur if such GWs are produced
during inflation or some post-inflation phase transition. In either case,
current observations provide a constraint to the GW amplitude that competes
with that from big-bang nucleosynthesis (BBN), although it extends to much
lower frequencies (~10^{-15} Hz rather than the ~10^{-10} Hz lower limit from
BBN): at 95% confidence-level, Omega_gw h^2 < 6.9 x 10^{-6} for homogeneous
(i.e., non-adiabatic) initial conditions. Future CMB experiments, like Planck
and CMBPol, should allow sensitivities to Omega_gw h^2 < 1.4 x 10^{-6} and
Omega_gw h^2 < 5 x 10^{-7}, respectively.Comment: 5 pages, 2 figures, submitted to Phys. Rev. Let
Spin Exchange Rates in Electron-Hydrogen Collisions
The spin temperature of neutral hydrogen, which determines the 21 cm optical
depth and brightness temperature, is set by the competition between radiative
and collisional processes. In the high-redshift intergalactic medium, the
dominant collisions are typically those between hydrogen atoms. However,
collisions with electrons couple much more efficiently to the spin state of
hydrogen than do collisions with other hydrogen atoms and thus become important
once the ionized fraction exceeds ~1%. Here we compute the rate at which
electron-hydrogen collisions change the hydrogen spin. Previous calculations
included only S-wave scattering and ignored resonances near the n=2 threshold.
We provide accurate results, including all partial wave terms through the
F-wave, for the de-excitation rate at temperatures T_K < 15,000 K; beyond that
point, excitation to n>=2 hydrogen levels becomes significant. Accurate
electron-hydrogen collision rates at higher temperatures are not necessary,
because collisional excitation in this regime inevitably produces Lyman-alpha
photons, which in turn dominate spin exchange when T_K > 6200 K even in the
absence of radiative sources. Our rates differ from previous calculations by
several percent over the temperature range of interest. We also consider some
simple astrophysical examples where our spin de-excitation rates are useful.Comment: submitted to MNRAS, 9 pages, 5 figure
Power spectrum normalization from the local abundance of rich clusters of galaxies
The number density of rich galaxy clusters still provides the most robust way
of normalizing the power spectrum of dark matter perturbations on scales
relevant to large-scale structure. We revisit this constraint in light of
several recent developments: (1) the availability of well-defined samples of
local clusters with relatively accurate X-ray temperatures; (2) new theoretical
mass functions for dark matter haloes which provide a good fit to large
numerical simulations; (3) more accurate mass-temperature relations from larger
catalogs of hydrodynamical simulations; (4) the requirement to consider closed
as well as open and flat cosmologies to obtain full multi-parameter likelihood
constraints for CMB and SNe studies. We present a new sample of clusters drawn
from the literature and use this sample to obtain improved results on sigma_8,
the normalization of the matter power spectrum on scales of 8 h^{-1} Mpc, as a
function of the matter density and cosmological constant in a Universe with
general curvature. We discuss our differences with previous work, and the
remaining major sources of uncertainty. Final results on the 68 per cent
confidence region, approximately independent of power spectrum shape, can be
expressed as constraints on sigma at an appropriate cluster normalization scale
R_Cl. We provide fitting formulas for R_Cl and sigma(R_Cl) for general
cosmologies, as well as for sigma_8 as a function of cosmology and shape
parameter Gamma. For flat models we find approximately sigma_8 \simeq
0.495^{+0.034}_{-0.037}) Omega_M^{-0.60} for Gamma=0.23, where the error bar is
dominated by uncertainty in the mass-temperature relation.Comment: 13 pages, minor changes in order to match the MNRAS published versio
Cosmological Signatures of Interacting Neutrinos
We investigate signatures of neutrino scattering in the Cosmic Microwave
Background (CMB) and matter power spectra, and the extent to which present
cosmological data can distinguish between a free streaming or tightly coupled
fluid of neutrinos. If neutrinos have strong non-standard interactions, for
example, through the coupling of neutrinos to a light boson, they may be kept
in equilibrium until late times. We show how the power spectra for these models
differ from more conventional neutrino scenarios, and use CMB and large scale
structure data to constrain these models. CMB polarization data improves the
constraints on the number of massless neutrinos, while the Lyman--
power spectrum improves the limits on the neutrino mass. Neutrino mass limits
depend strongly on whether some or all of the neutrino species interact and
annihilate. The present data can accommodate a number of tightly-coupled
relativistic degrees of freedom, and none of the interacting-neutrino scenarios
considered are ruled out by current data -- although considerations regarding
the age of the Universe disfavor a model with three annihilating neutrinos with
very large neutrino masses.Comment: 17 pages, 14 figures, minor changes and references added, published
in Phys. Rev.
The 3D soft X-ray cluster-AGN cross-correlation function in the ROSAT NEP survey
X-ray surveys facilitate investigations of the environment of AGNs. Deep
Chandra observations revealed that the AGNs source surface density rises near
clusters of galaxies. The natural extension of these works is the measurement
of spatial clustering of AGNs around clusters and the investigation of relative
biasing between active galactic nuclei and galaxies near clusters.The major
aims of this work are to obtain a measurement of the correlation length of AGNs
around clusters and a measure of the averaged clustering properties of a
complete sample of AGNs in dense environments. We present the first measurement
of the soft X-ray cluster-AGN cross-correlation function in redshift space
using the data of the ROSAT-NEP survey. The survey covers 9x9 deg^2 around the
North Ecliptic Pole where 442 X-ray sources were detected and almost completely
spectroscopically identified. We detected a >3sigma significant clustering
signal on scales s<50 h70^-1 Mpc. We performed a classical maximum-likelihood
power-law fit to the data and obtained a correlation length s_0=8.7+1.2-0.3
h_70-1 Mpc and a slope gamma=1.7$^+0.2_-0.7 (1sigma errors). This is a strong
evidence that AGNs are good tracers of the large scale structure of the
Universe. Our data were compared to the results obtained by cross-correlating
X-ray clusters and galaxies. We observe, with a large uncertainty, that the
bias factor of AGN is similar to that of galaxies.Comment: 4 pages, 2 figure, proceedings of the Conference "At the edge of the
Universe", Sintra Portugal, October 2006. To be published on the Astronomical
Society of the Pacific Conference Series (ASPCS
The initial conditions of the universe: how much isocurvature is allowed?
We investigate the constraints imposed by the current data on correlated
mixtures of adiabatic and non-adiabatic primordial perturbations. We discover
subtle flat directions in parameter space that tolerate large (~60%)
contributions of non-adiabatic fluctuations. In particular, larger values of
the baryon density and a spectral tilt are allowed. The cancellations in the
degenerate directions are explored and the role of priors elucidated.Comment: 4 pages, 4 figures. Submitted to PR
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