3,185 research outputs found
A Bayesian estimate of the CMB-large-scale structure cross-correlation
Evidences for late-time acceleration of the Universe are provided by multiple
probes, such as Type Ia supernovae, the cosmic microwave background (CMB) and
large-scale structure (LSS). In this work, we focus on the integrated
Sachs--Wolfe (ISW) effect, i.e., secondary CMB fluctuations generated by
evolving gravitational potentials due to the transition between, e.g., the
matter and dark energy (DE) dominated phases. Therefore, assuming a flat
universe, DE properties can be inferred from ISW detections. We present a
Bayesian approach to compute the CMB--LSS cross-correlation signal. The method
is based on the estimate of the likelihood for measuring a combined set
consisting of a CMB temperature and a galaxy contrast maps, provided that we
have some information on the statistical properties of the fluctuations
affecting these maps. The likelihood is estimated by a sampling algorithm,
therefore avoiding the computationally demanding techniques of direct
evaluation in either pixel or harmonic space. As local tracers of the matter
distribution at large scales, we used the Two Micron All Sky Survey (2MASS)
galaxy catalog and, for the CMB temperature fluctuations, the ninth-year data
release of the Wilkinson Microwave Anisotropy Probe (WMAP9). The results show a
dominance of cosmic variance over the weak recovered signal, due mainly to the
shallowness of the catalog used, with systematics associated with the sampling
algorithm playing a secondary role as sources of uncertainty. When combined
with other complementary probes, the method presented in this paper is expected
to be a useful tool to late-time acceleration studies in cosmology.Comment: 21 pages, 15 figures, 4 tables. We extended the previous analyses
including WMAP9 Q, V and W channels, besides the ILC map. Updated to match
accepted ApJ versio
SILC: a new Planck Internal Linear Combination CMB temperature map using directional wavelets
We present new clean maps of the CMB temperature anisotropies (as measured by
Planck) constructed with a novel internal linear combination (ILC) algorithm
using directional, scale-discretised wavelets --- Scale-discretised,
directional wavelet ILC or SILC. Directional wavelets, when convolved with
signals on the sphere, can separate the anisotropic filamentary structures
which are characteristic of both the CMB and foregrounds. Extending previous
component separation methods, which use the frequency, spatial and harmonic
signatures of foregrounds to separate them from the cosmological background
signal, SILC can additionally use morphological information in the foregrounds
and CMB to better localise the cleaning algorithm. We test the method on Planck
data and simulations, demonstrating consistency with existing component
separation algorithms, and discuss how to optimise the use of morphological
information by varying the number of directional wavelets as a function of
spatial scale. We find that combining the use of directional and axisymmetric
wavelets depending on scale could yield higher quality CMB temperature maps.
Our results set the stage for the application of SILC to polarisation
anisotropies through an extension to spin wavelets.Comment: 15 pages, 13 figures. Minor changes to match version published in
MNRAS. Map products available at http://www.silc-cmb.or
A Large Sky Simulation of the Gravitational Lensing of the Cosmic Microwave Background
Large scale structure deflects cosmic microwave background (CMB) photons.
Since large angular scales in the large scale structure contribute
significantly to the gravitational lensing effect, a realistic simulation of
CMB lensing requires a sufficiently large sky area. We describe simulations
that include these effects, and present both effective and multiple plane
ray-tracing versions of the algorithm, which employs spherical harmonic space
and does not use the flat sky approximation. We simulate lensed CMB maps with
an angular resolution of ~0.9 arcmin. The angular power spectrum of the
simulated sky agrees well with analytical predictions. Maps generated in this
manner are a useful tool for the analysis and interpretation of upcoming CMB
experiments such as PLANCK and ACT.Comment: 14 pages, 12 figures, replaced with version accepted for publication
by the AP
Bayesian inference on the sphere beyond statistical isotropy
We present a general method for Bayesian inference of the underlying
covariance structure of random fields on a sphere. We employ the Bipolar
Spherical Harmonic (BipoSH) representation of general covariance structure on
the sphere. We illustrate the efficacy of the method as a principled approach
to assess violation of statistical isotropy (SI) in the sky maps of Cosmic
Microwave Background (CMB) fluctuations. SI violation in observed CMB maps
arise due to known physical effects such as Doppler boost and weak lensing; yet
unknown theoretical possibilities like cosmic topology and subtle violations of
the cosmological principle, as well as, expected observational artefacts of
scanning the sky with a non-circular beam, masking, foreground residuals,
anisotropic noise, etc. We explicitly demonstrate the recovery of the input SI
violation signals with their full statistics in simulated CMB maps. Our
formalism easily adapts to exploring parametric physical models with non-SI
covariance, as we illustrate for the inference of the parameters of a Doppler
boosted sky map. Our approach promises to provide a robust quantitative
evaluation of the evidence for SI violation related anomalies in the CMB sky by
estimating the BipoSH spectra along with their complete posterior.Comment: 16 pages, 6 figure
The actual Rees--Sciama effect from the Local Universe
Observations of the Cosmic Microwave Background (CMB) have revealed an
unexpected quadrupole-octopole alignment along a preferred axis pointing toward
the Virgo cluster. We here investigate whether this feature can be explained in
the framework of the concordance model by secondary anisotropies produced by
the non-linear evolution of the gravitational potential, the so-called
Rees-Sciama (RS) effect. We focus on the effect caused by the local
superclusters, which we calculate using a constrained high-resolution
hydrodynamical simulation, based on the IRAS 1.2-Jy all-sky galaxy redshift
survey, which reproduces the main structures of our Universe out to a distance
of 110 Mpc from our Galaxy. The resulting RS effect peaks at low multipoles and
has a minimum/maximum amplitude of -6.6\mu K 1.9\mu K. Even though its
quadrupole is well aligned with the one measured for the CMB, its amplitude is
not sufficient to explain the observed magnitude of the quadrupole/octopole
alignment. In addition, we analyze the WMAP-3 data with a linear matched filter
in an attempt to determine an upper limit for the RS signal amplitude on large
scales. We found that it is possible to infer a weak upper limit of 30\mu K for
its maximum amplitude.Comment: 7 pages, 4 figures, submitted to A&
Statistical Properties of Galactic Starlight Polarization
We present a statistical analysis of Galactic interstellar polarization from
the largest compilation available of starlight data. The data comprises ~ 9300
stars of which we have selected ~ 5500 for our analysis. We find a nearly
linear growth of mean polarization degree with extinction. The amplitude of
this correlation shows that interstellar grains are not fully aligned with the
Galactic magnetic field, which can be interpreted as the effect of a large
random component of the field. In agreement with earlier studies of more
limited scope, we estimate the ratio of the uniform to the random
plane-of-the-sky components of the magnetic field to be B_u/B_r = 0.8.
Moreover, a clear correlation exists between polarization degree and
polarization angle what provides evidence that the magnetic field geometry
follows Galactic structures on large-scales. The angular power spectrum C_l of
the starlight polarization degree for Galactic plane data (|b| < 10 deg) is
consistent with a power-law, C_l ~ l^{-1.5} (where l ~ 180 deg/\theta is the
multipole order), for all angular scales \theta > 10 arcmin. An investigation
of sparse and inhomogeneous sampling of the data shows that the starlight data
analyzed traces an underlying polarized continuum that has the same power
spectrum slope, C_l ~ l^{-1.5}. Our findings suggest that starlight data can be
safely used for the modeling of Galactic polarized continuum emission at other
wavelengths.Comment: 31 pages, 11 figures. Minor corrections and some clarifications
included. Matches version accepted for publication by the Astrophysical
Journa
A multifrequency angular power spectrum analysis of the Leiden polarization surveys
The Galactic synchrotron emission is expected to be the most relevant source
of astrophysical contamination in cosmic microwave background polarization
measurements, at least at frequencies 30'. We
present a multifrequency analysis of the Leiden surveys, linear polarization
surveys covering essentially the Northern Celestial Hemisphere at five
frequencies between 408 MHz and 1411 MHz. By implementing specific
interpolation methods to deal with these irregularly sampled data, we produced
maps of the polarized diffuse Galactic radio emission with pixel size of 0.92
deg. We derived the angular power spectrum (APS) (PI, E, and B modes) of the
synchrotron dominated radio emission as function of the multipole, l. We
considered the whole covered region and some patches at different Galactic
latitudes. By fitting the APS in terms of power laws (C_l = k l^a), we found
spectral indices that steepen with increasing frequency: from a = -(1-1.5) at
408 MHz to a = -(2-3) at 1411 MHz for 10 < l < 100 and from a = -0.7 to a =
-1.5 for lower multipoles (the exact values depending on the considered sky
region and polarization mode). The bulk of this steepening can be interpreted
in terms of Faraday depolarization effects. We then considered the APS at
various fixed multipoles and its frequency dependence. Using the APSs of the
Leiden surveys at 820 MHz and 1411 MHz, we determined possible ranges for the
rotation measure, RM, in the simple case of an interstellar medium slab model.
Taking also into account the polarization degree at 1.4 GHz, we could break the
degeneracy between the identified RM intervals. The most reasonable of them
turned out to be RM = 9-17 rad/m^2.Comment: 18 pages, 14 figures. Astronomy and Astrophysics, in pres
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