123 research outputs found
Characterization of the non-Gaussianity of radio and IR point sources at CMB frequencies
This study, using publicly available simulations, focuses on the characterization of the non-Gaussianity produced by radio point sources and by infrared (IR) sources in the frequency range of the cosmic microwave background from 30 to 350 GHz. We propose a simple prescription to infer the angular bispectrum from the power spectrum of point sources considering independent populations of sources, with or without clustering. We test the accuracy of our prediction using publicly available all-sky simulations of radio and IR sources and find very good agreement. We further characterize the configuration dependence and the frequency behaviour of the IR and radio bispectra. We show that the IR angular bispectrum peaks for squeezed triangles and that the clustering of IR sources enhances the bispectrum values by several orders of magnitude at scales ℓ∼ 100. At 150 GHz the bispectrum of IR sources starts to dominate that of radio sources on large angular scales, and it dominates the whole multipole range at 350 GHz. Finally, we compute the bias on fNL induced by radio and IR sources. We show that the positive bias induced by radio sources is significantly reduced by masking the sources. We also show, for the first time, that the form of the IR bispectrum mimics a primordial ‘local' bispectrum fNL. The IR sources produce a negative bias which becomes important for Planck-like resolution and at high frequencies (ΔfNL∼−6 at 277 GHz and ΔfNL∼−60-70 at 350 GHz). Most of the signal being due to the clustering of faint IR sources, the bias is not reduced by masking sources above a flux limit and may, in some cases, even be increased due to the reduction of the shot-noise ter
Large scale directional anomalies in the WMAP 5yr ILC map
We study the alignments of the low multipoles of CMB anisotropies with
specific directions in the sky (i.e. the dipole, the north Ecliptic pole, the
north Galactic pole and the north Super Galactic pole). Performing
random extractions we have found that: 1) separately quadrupole and octupole
are mildly orthogonal to the dipole but when they are considered together, in
analogy to \cite{Copi2006}, we find an unlikely orthogonality at the level of
0.8% C.L.; 2) the multipole vectors associated to are unlikely aligned
with the dipole at C.L.; 3) the multipole vectors associated to
are mildly orthogonal to the dipole but when we consider only maps
that show exactly the same correlation among the multipoles as in the observed
WMAP 5yr ILC, these multipole vectors are unlikely orthogonal to the dipole at
C.L..Comment: 12 pages, 10 figures, 3 tables. Accepted for publication in JCAP. Few
references added and some typos correcte
The Kolmogorov-Smirnov test for the CMB
We investigate the statistics of the cosmic microwave background using the
Kolmogorov-Smirnov test. We show that, when we correctly de-correlate the data,
the partition function of the Kolmogorov stochasticity parameter is compatible
with the Kolmogorov distribution and, contrary to previous claims, the CMB data
are compatible with Gaussian fluctuations with the correlation function given
by standard Lambda-CDM. We then use the Kolmogorov-Smirnov test to derive upper
bounds on residual point source power in the CMB, and indicate the promise of
this statistics for further datasets, especially Planck, to search for
deviations from Gaussianity and for detecting point sources and Galactic
foregrounds.Comment: Improved significance of the results (which remain unchanged) by
using patches instead of ring segments in the analysis. Added sky maps of the
Kolmogorov-parameter for original and de-correlated CMB ma
Efficient cosmological parameter sampling using sparse grids
We present a novel method to significantly speed up cosmological parameter
sampling. The method relies on constructing an interpolation of the
CMB-log-likelihood based on sparse grids, which is used as a shortcut for the
likelihood-evaluation. We obtain excellent results over a large region in
parameter space, comprising about 25 log-likelihoods around the peak, and we
reproduce the one-dimensional projections of the likelihood almost perfectly.
In speed and accuracy, our technique is competitive to existing approaches to
accelerate parameter estimation based on polynomial interpolation or neural
networks, while having some advantages over them. In our method, there is no
danger of creating unphysical wiggles as it can be the case for polynomial fits
of a high degree. Furthermore, we do not require a long training time as for
neural networks, but the construction of the interpolation is determined by the
time it takes to evaluate the likelihood at the sampling points, which can be
parallelised to an arbitrary degree. Our approach is completely general, and it
can adaptively exploit the properties of the underlying function. We can thus
apply it to any problem where an accurate interpolation of a function is
needed.Comment: Submitted to MNRAS, 13 pages, 13 figure
CMB polarization as a probe of the anomalous nature of the Cold Spot
One of the most interesting explanations for the non-Gaussian Cold Spot (CS)
detected in the WMAP data by Vielva et al. 2004, is that it arises from the
interaction of the CMB radiation with a cosmic texture (Cruz et al. 2007b). In
this case, a lack of polarization is expected in the region of the spot, as
compared to the typical values associated to large fluctuations of a GIRF. In
addition, other physical processes related to a non-linear evolution of the
gravitational field could lead to a similar scenario. However, some of these
alternative scenarios (e.g., a large void in the large scale structure) have
been shown to be very unlikely. In this work we characterise the polarization
properties of the Cold Spot under both hypotheses: a large Gaussian spot and an
anomalous feature generated, for instance, by a cosmic texture. We propose a
methodology to distinguish between them, and we discuss its discrimination
power as a function of the instrumental noise level. In particular, we address
the cases of current experiments, like WMAP and Planck, and others in
development as QUIJOTE. We find that for an ideal experiment the Gaussian
hypothesis could be rejected at a significance level better than 0.8%. While
WMAP is far from providing useful information in this respect, we find that
Planck will be able to reach a significance of around 7%; in addition, we show
that the ground-based experiment QUIJOTE could provide a significance of around
1%. If these results are combined with the significance level found for the CS
in temperature, the capability of QUIJOTE and Planck to reject the alternative
hypothesis becomes 0.025% and 0.124%, respectively.Comment: 7 pages, 3 figures, accepted in MNRAS. Minor changes made to match
the final versio
Anomalous variance in the WMAP data and Galactic Foreground residuals
A previous work (Monteser\'in et al. 2008) estimated the CMB variance from
the three-year WMAP data, finding a lower value than expected from Gaussian
simulations using the WMAP best-fit cosmological model. We repeat the analysis
on the five-year WMAP data using a new estimator with lower bias and variance.
Our results confirm this anomaly at higher significance, namely with a p-value
of 0.31%. We perform the analysis using different exclusion masks, showing that
a particular region of the sky near the Galactic plane shows a higher variance
than 95.58% of the simulations whereas the rest of the sky has a lower variance
than 99.96% of the simulations. The relative difference in variance between
both regions is bigger than in 99.64% of the simulations. This anisotropic
distribution of power seems to be causing the anomaly since the model assumes
isotropy. Furthermore, this region has a clear frequency dependence between
41GHz and 61GHz or 94GHz suggesting that Galactic foreground residuals could be
responsible for the anomaly. Moreover, removing the quadrupole and the octopole
from data and simulations the anomaly disappears. The variance anomaly and the
previously reported quadrupole and octopole alignment seem therefore to be
related and could have a common origin. We discuss different possible causes
and Galactic foreground residuals seem to be the most likely one. These
residuals would affect the estimation of the angular power spectrum from the
WMAP data, which is used to generate Gaussian simulations, giving rise to an
inconsistency between the estimated and expected CMB variance. If the presence
of residuals is confirmed, the estimation of the cosmological parameters could
be affected.Comment: Accepted for publication in MNRAS. Analysis section rewritten. New
exclusion masks are used finding a high variance region. Relation to the
Quadrupole-Octopole alignment foun
Searching for a Cosmological Preferred Axis: Union2 Data Analysis and Comparison with Other Probes
We review, compare and extend recent studies searching for evidence for a
preferred cosmological axis. We start from the Union2 SnIa dataset and use the
hemisphere comparison method to search for a preferred axis in the data. We
find that the hemisphere of maximum accelerating expansion rate is in the
direction (\omm=0.19) while the hemisphere of
minimum acceleration is in the opposite direction
(\omm=0.30). The level of anisotropy is described by the normalized
difference of the best fit values of \omm between the two hemispheres in the
context of \lcdm fits. We find a maximum anisotropy level in the Union2 data of
\frac{\Delta \ommax}{\bomm}=0.43\pm 0.06. Such a level does not necessarily
correspond to statistically significant anisotropy because it is reproduced by
about of simulated isotropic data mimicking the best fit Union2 dataset.
However, when combined with the axes directions of other cosmological
observations (bulk velocity flow axis, three axes of CMB low multipole moments
and quasar optical polarization alignment axis), the statistical evidence for a
cosmological anisotropy increases dramatically. We estimate the probability
that the above independent six axes directions would be so close in the sky to
be less than . Thus either the relative coincidence of these six axes is a
very large statistical fluctuation or there is an underlying physical or
systematic reason that leads to their correlation.Comment: 10 pages, 7 figures. Accepted in JCAP (to appear). Extended analysis
with redshift tomography of SnIa, included errorbars and increased number of
axes. The Mathematica 7 files with the data used for the production of the
figures along with a Powerpoint file with additional figures may be
downloaded from http://leandros.physics.uoi.gr/anisotrop
Planck Intermediate Results. IV. The XMM-Newton validation programme for new Planck galaxy clusters
We present the final results from the XMM-Newton validation follow-up of new
Planck galaxy cluster candidates. We observed 15 new candidates, detected with
signal-to-noise ratios between 4.0 and 6.1 in the 15.5-month nominal Planck
survey. The candidates were selected using ancillary data flags derived from
the ROSAT All Sky Survey (RASS) and Digitized Sky Survey all-sky maps, with the
aim of pushing into the low SZ flux, high-z regime and testing RASS flags as
indicators of candidate reliability. 14 new clusters were detected by XMM,
including 2 double systems. Redshifts lie in the range 0.2 to 0.9, with 6
clusters at z>0.5. Estimated M500 range from 2.5 10^14 to 8 10^14 Msun. We
discuss our results in the context of the full XMM validation programme, in
which 51 new clusters have been detected. This includes 4 double and 2 triple
systems, some of which are chance projections on the sky of clusters at
different z. We find that association with a RASS-BSC source is a robust
indicator of the reliability of a candidate, whereas association with a FSC
source does not guarantee that the SZ candidate is a bona fide cluster.
Nevertheless, most Planck clusters appear in RASS maps, with a significance
greater than 2 sigma being a good indication that the candidate is a real
cluster. The full sample gives a Planck sensitivity threshold of Y500 ~ 4 10^-4
arcmin^2, with indication for Malmquist bias in the YX-Y500 relation below this
level. The corresponding mass threshold depends on z. Systems with M500 > 5
10^14 Msun at z > 0.5 are easily detectable with Planck. The newly-detected
clusters follow the YX-Y500 relation derived from X-ray selected samples.
Compared to X-ray selected clusters, the new SZ clusters have a lower X-ray
luminosity on average for their mass. There is no indication of departure from
standard self-similar evolution in the X-ray versus SZ scaling properties.
(abridged)Comment: accepted by A&
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