168,016 research outputs found
Reconstructing Projected Matter Density from Cosmic Microwave Background
Gravitational lensing distorts the cosmic microwave background (CMB)
anisotropies and imprints a characteristic pattern onto it. The distortions
depend on the projected matter density between today and redshift . In this paper we develop a method for a direct reconstruction of the
projected matter density from the CMB anisotropies. This reconstruction is
obtained by averaging over quadratic combinations of the derivatives of CMB
field. We test the method using simulations and show that it can successfully
recover projected density profile of a cluster of galaxies if there are
measurable anisotropies on scales smaller than the characteristic cluster size.
In the absence of sufficient small scale power the reconstructed maps have low
signal to noise on individual structures, but can give a positive detection of
the power spectrum or when cross correlated with other maps of large scale
structure. We develop an analytic method to reconstruct the power spectrum
including the effects of noise and beam smoothing. Tests with Monte Carlo
simulations show that we can recover the input power spectrum both on large and
small scales, provided that we use maps with sufficiently low noise and high
angular resolution.Comment: 21 pages, 9 figures, submitted to PR
gamma-ray DBSCAN: a clustering algorithm applied to Fermi-LAT gamma-ray data. I. Detection performances with real and simulated data
The Density Based Spatial Clustering of Applications with Noise (DBSCAN) is a
topometric algorithm used to cluster spatial data that are affected by
background noise. For the first time, we propose the use of this method for the
detection of sources in gamma-ray astrophysical images obtained from the
Fermi-LAT data, where each point corresponds to the arrival direction of a
photon. We investigate the detection performance of the gamma-ray DBSCAN in
terms of detection efficiency and rejection of spurious clusters, using a
parametric approach, and exploring a large volume of the gamma-ray DBSCAN
parameter space. By means of simulated data we statistically characterize the
gamma-ray DBSCAN, finding signatures that differentiate purely random fields,
from fields with sources. We define a significance level for the detected
clusters, and we successfully test this significance with our simulated data.
We apply the method to real data, and we find an excellent agreement with the
results obtained with simulated data. We find that the gamma-ray DBSCAN can be
successfully used in the detection of clusters in gamma-ray data. The
significance returned by our algorithm is strongly correlated with that
provided by the Maximum Likelihood analysis with standard Fermi-LAT software,
and can be used to safely remove spurious clusters. The positional accuracy of
the reconstructed cluster centroid compares to that returned by standard
Maximum Likelihood analysis, allowing to look for astrophysical counterparts in
narrow regions, minimizing the chance probability in the counterpart
association. We find that gamma-ray DBSCAN is a powerful tool in the detection
of clusters in gamma-ray data, this method can be used both to look for
point-like sources, and extended sources, and can be potentially applied to any
astrophysical field related with detection of clusters in data.Comment: Accepted for publication in A&
Bayesian modelling of clusters of galaxies from multi-frequency pointed Sunyaev--Zel'dovich observations
We present a Bayesian approach to modelling galaxy clusters using
multi-frequency pointed observations from telescopes that exploit the
Sunyaev--Zel'dovich effect. We use the recently developed MultiNest technique
(Feroz, Hobson & Bridges, 2008) to explore the high-dimensional parameter
spaces and also to calculate the Bayesian evidence. This permits robust
parameter estimation as well as model comparison. Tests on simulated Arcminute
Microkelvin Imager observations of a cluster, in the presence of primary CMB
signal, radio point sources (detected as well as an unresolved background) and
receiver noise, show that our algorithm is able to analyse jointly the data
from six frequency channels, sample the posterior space of the model and
calculate the Bayesian evidence very efficiently on a single processor. We also
illustrate the robustness of our detection process by applying it to a field
with radio sources and primordial CMB but no cluster, and show that indeed no
cluster is identified. The extension of our methodology to the detection and
modelling of multiple clusters in multi-frequency SZ survey data will be
described in a future work.Comment: 12 pages, 7 figures, submitted to MNRA
On the ISW-cluster cross-correlation in future surveys
We investigate the cosmological information contained in the
cross-correlation between the Integrated Sachs-Wolfe (ISW) of the Cosmic
Microwave Background (CMB) anisotropy pattern and galaxy clusters from future
wide surveys. Future surveys will provide cluster catalogues with a number of
objects comparable with galaxy catalogues currently used for the detection of
the ISW signal by cross-correlation with the CMB anisotropy pattern. By
computing the angular power spectra of clusters and the corresponding
cross-correlation with CMB, we perform a signal-to-noise ratio (SNR) analysis
for the ISW detection as expected from the eROSITA and the Euclid space
missions. We discuss the dependence of the SNR of the ISW-cluster
cross-correlation on the specifications of the catalogues and on the reference
cosmology. We forecast that the SNRs for ISW-cluster cross-correlation are
alightly smaller compared to those which can be obtained from future galaxy
surveys but the signal is expected to be detected at high significance, i.e.
more than . We also forecast the joint constraints on parameters
of model extensions of the concordance CDM cosmology by combining CMB
and the ISW-cluster cross-correlation.Comment: 12 pages, 10 figures. Matches version accepted in MNRA
Detecting Sunyaev-Zel'dovich clusters with PLANCK: III. Properties of the expected SZ-cluster sample
The PLANCK-mission is the most sensitive all-sky submillimetric mission
currently being planned and prepared. Special emphasis is given to the
observation of clusters of galaxies by their thermal Sunyaev-Zel'dovich (SZ)
effect. In this work, the results of a simulation are presented that combines
all-sky maps of the thermal and kinetic SZ-effect with cosmic microwave
background (CMB) fluctuations, Galactic foregrounds (synchrotron emission,
thermal emission from dust, free-free emission and rotational transitions of
carbon monoxide molecules) and sub-millimetric emission from planets and
asteroids of the Solar System. Observational issues, such as PLANCKs beam
shapes, frequency response and spatially non-uniform instrumental noise have
been incorporated. Matched and scale-adaptive multi-frequency filtering schemes
have been extended to spherical coordinates and are now applied to the data
sets in order to isolate and amplify the weak thermal SZ-signal. The properties
of the resulting SZ-cluster sample are characterised in detail: Apart from the
number of clusters as a function of cluster parameters such as redshift z and
total mass M, the distribution n(sigma)d sigma of the detection significance
sigma, the number of detectable clusters in relation to the model cluster
parameters entering the filter construction, the position accuracy of an
SZ-detection and the cluster number density as a function of ecliptic latitude
beta is examined.Comment: 14 pages, 16 figures, 13 tables, submitted to MNRAS, 16.Feb.200
Ringing effects reduction by improved deconvolution algorithm Application to A370 CFHT image of gravitational arcs
We develop a self-consistent automatic procedure to restore informations from
astronomical observations. It relies on both a new deconvolution algorithm
called LBCA (Lower Bound Constraint Algorithm) and the use of the Wiener
filter. In order to explore its scientific potential for strong and weak
gravitational lensing, we process a CFHT image of the galaxies cluster Abell
370 which exhibits spectacular strong gravitational lensing effects. A high
quality restoration is here of particular interest to map the dark matter
within the cluster. We show that the LBCA turns out specially efficient to
reduce ringing effects introduced by classical deconvolution algorithms in
images with a high background. The method allows us to make a blind detection
of the radial arc and to recover morphological properties similar to
thoseobserved from HST data. We also show that the Wiener filter is suitable to
stop the iterative process before noise amplification, using only the
unrestored data.Comment: A&A in press 9 pages 9 figure
Point Source Confusion in SZ Cluster Surveys
We examine the effect of point source confusion on cluster detection in
Sunyaev-Zel'dovich (SZ) surveys. A filter matched to the spatial and spectral
characteristics of the SZ signal optimally extracts clusters from the
astrophysical backgrounds. We calculate the expected confusion (point source
and primary cosmic microwave background [CMB]) noise through this filter and
quantify its effect on the detection threshold for both single and multiple
frequency surveys. Extrapolating current radio counts, we estimate that
confusion from sources below 100 microJy limits single-frequency surveys to
1-sigma detection thresholds of Y 3.10^{-6} arcmin^2 at 30 GHz and Y 10^{-5}
arcmin^2 at 15 GHz (for unresolved clusters in a 2 arcmin beam); these numbers
are highly uncertain, and an extrapolation with flatter counts leads to much
lower confusion limits. Bolometer surveys must contend with an important
population of infrared point sources. We find that a three-band matched filter
with 1 arcminute resolution (in each band) efficiently reduces confusion, but
does not eliminate it: residual point source and CMB fluctuations contribute
significantly the total filter noise. In this light, we find that a 3-band
filter with a low-frequency channel (e.g, 90+150+220 GHz) extracts clusters
more effectively than one with a high frequency channel (e.g, 150+220+300 GHz).Comment: Accepted for publication in Astronomy & Astrophysics; Updated grant
information in acknowledgement
Detection of Enhancement in Number Densities of Background Galaxies due to Magnification by Massive Galaxy Clusters
We present a detection of the enhancement in the number densities of
background galaxies induced from lensing magnification and use it to test the
Sunyaev-Zel'dovich effect (SZE) inferred masses in a sample of 19 galaxy
clusters with median redshift selected from the South Pole
Telescope SPT-SZ survey. Two background galaxy populations are selected for
this study through their photometric colours; they have median redshifts
(low- background) and
(high- background). Stacking these
populations, we detect the magnification bias effect at and
for the low- and high- backgrounds, respectively. We fit NFW
models simultaneously to all observed magnification bias profiles to estimate
the multiplicative factor that describes the ratio of the weak lensing
mass to the mass inferred from the SZE observable-mass relation. We further
quantify systematic uncertainties in resulting from the photometric
noise and bias, the cluster galaxy contamination and the estimations of the
background properties. The resulting for the combined background
populations with uncertainties is
, indicating good consistency
between the lensing and the SZE-inferred masses. We use our best-fit to
predict the weak lensing shear profiles and compare these predictions with
observations, showing agreement between the magnification and shear mass
constraints. This work demonstrates the promise of using the magnification as a
complementary method to estimate cluster masses in large surveys.Comment: 16 pages, 10 figures, accepted for publication in MNRA
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