220 research outputs found
A Bayesian approach to filter design: detection of compact sources
We consider filters for the detection and extraction of compact sources on a
background. We make a one-dimensional treatment (though a generalization to two
or more dimensions is possible) assuming that the sources have a Gaussian
profile whereas the background is modeled by an homogeneous and isotropic
Gaussian random field, characterized by a scale-free power spectrum. Local peak
detection is used after filtering. Then, a Bayesian Generalized Neyman-Pearson
test is used to define the region of acceptance that includes not only the
amplification but also the curvature of the sources and the a priori
probability distribution function of the sources. We search for an optimal
filter between a family of Matched-type filters (MTF) modifying the filtering
scale such that it gives the maximum number of real detections once fixed the
number density of spurious sources. We have performed numerical simulations to
test theoretical ideas.Comment: 10 pages, 2 figures. SPIE Proceedings "Electronic Imaging II", San
Jose, CA. January 200
Non-blind catalogue of extragalactic point sources from the Wilkinson Microwave Anisotropy Probe (WMAP) first 3--year survey data
We have used the MHW2 filter to obtain estimates of the flux densities at the
WMAP frequencies of a complete sample of 2491 sources, mostly brighter than 500
mJy at 5 GHz, distributed over the whole sky excluding a strip around the
Galactic equator (b < 5 degrees). After having detected 933 sources above the 3
sigma level in the MHW2 filtered maps - our New Extragalactic WMAP Point Source
(NEWPS_3sigma) Catalogue - we are left with 381 sources above 5 sigma in at
least one WMAP channel, 369 of which constitute our NEWPS_5sigma catalogue. It
is remarkable to note that 98 (i.e. 26%) sources detected above 5 sigma are
`new', they are not present in the WMAP catalogue. Source fluxes have been
corrected for the Eddington bias. Our flux density estimates before such
correction are generally in good agreement with the WMAP ones at 23 GHz. At
higher frequencies WMAP fluxes tend to be slightly higher than ours, probably
because WMAP estimates neglect the deviations of the point spread function from
a Gaussian shape. On the whole, above the estimated completeness limit of 1.1
Jy at 23 GHz we detected 43 sources missed by the blind method adopted by the
WMAP team. On the other hand, our low-frequency selection threshold left out 25
WMAP sources, only 12 of which, however, are 5 sigma detections and only 3 have
fluxes S at 23 GHz > 1.1 Jy. Thus, our approach proved to be competitive with,
and complementary to the WMAP one.Comment: 18 pages, 6 figures, 5 tables. Accepted for publication in ApJ
Detection of point sources on two-dimensional images based on peaks
This article considers the detection of point sources in two dimensional
astronomical images. The detection scheme we propose is based on peak
statistics. We discuss the example of the detection of far galaxies in Cosmic
Microwave Background experiments throughout the paper, although the method we
present is totally general and can be used in many other fields of data
analysis. We assume sources with a Gaussian profile --that is a fair
approximation of the profile of a point source convolved with the detector beam
in microwave experiments-- on a background modeled by a homogeneous and
isotropic Gaussian random field characterized by a scale-free power spectrum.
Point sources are enhanced with respect to the background by means of linear
filters. After filtering, we identify local maxima and apply our detection
scheme, a Neyman-Pearson detector that defines our region of acceptance based
on the a priori pdf of the sources and the ratio of number densities. We study
the different performances of some linear filters that have been used in this
context in the literature: the Mexican Hat wavelet, the matched filter and the
scale-adaptive filter. We consider as well an extension to two dimensions of
the biparametric scale adaptive filter (BSAF). The BSAF depends on two
parameters which are determined by maximizing the number density of real
detections while fixing the number density of spurious detections. For our
detection criterion the BSAF outperforms the other filters in the interesting
case of white noise.Comment: 21 pages, 3 figures, version accepted for publication on EURASIP
Journal on Applied Signal Processing: Applications of Signal Processing in
Astrophysics and Cosmolog
Beam-deconvolved Planck LFI maps
12 pages, 7 figuresThe Planck Collaboration made its final data release in 2018. In this paper we describe beam-deconvolution map products made from Planck LFI data using the artDeco deconvolution code to symmetrize the effective beam. The deconvolution results are auxiliary data products, available through the Planck Legacy Archive. Analysis of these deconvolved survey difference maps reveals signs of residual signal in the 30-GHz and 44-GHz frequency channels. We produce low-resolution maps and corresponding noise covariance matrices (NCVMs). The NCVMs agree reasonably well with the half-ring noise estimates except for 44\,GHz, where we observe an asymmetry between and noise spectra, possibly a sign of further unresolved systematics.Peer reviewe
Powellsnakes II: a fast Bayesian approach to discrete object detection in multi-frequency astronomical data sets
Powellsnakes is a Bayesian algorithm for detecting compact objects embedded
in a diffuse background, and was selected and successfully employed by the
Planck consortium in the production of its first public deliverable: the Early
Release Compact Source Catalogue (ERCSC). We present the critical foundations
and main directions of further development of PwS, which extend it in terms of
formal correctness and the optimal use of all the available information in a
consistent unified framework, where no distinction is made between point
sources (unresolved objects), SZ clusters, single or multi-channel detection.
An emphasis is placed on the necessity of a multi-frequency, multi-model
detection algorithm in order to achieve optimality
Selecting a complete sample of blazars in sub-millimetre catalogues
The Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS), which has covered about 642 sq. deg. in five bands from 100 to 500 mu m, allows a blind flux-limited selection of blazars at sub-mm wavelengths. However, blazars constitute a tiny fraction of H-ATLAS sources and therefore identifying them is not a trivial task. Using the data on known blazars detected by the H-ATLAS, we have defined a locus for 500 p.m selected blazars and exploited it to select blazar candidates in the H-ATLAS fields. Candidates and known blazars in the H-ATLAS equatorial and South Galactic Pole fields were followed up with the Australia Telescope Compact Array (ATCA) or with the Karl G. Jansky Very Large Array (VLA), and matched with existing radio- and mm-catalogues to reconstruct the spectral behaviour over at least six orders of magnitude in frequency. We identified a selection approach that, combining the information in the sub-mm and radio domains, efficiently singles out genuine blazars. In this way, we identified a sample of 39 blazars brighter than S-500 mu m = 35 mJy in the H-ATLAS fields. Tests made cross-matching the H-ATLAS catalogues with large catalogues of blazar candidates indicate that the sample is complete. The derived counts are compared with model predictions finding good consistency with the C2Ex model and with estimates based on ALMA data
Forecasts on the contamination induced by unresolved point sources in primordial non-Gaussianity beyond Planck
In this paper we present forecasts of the contamination on different shapes of the
primordial non-Gaussianity fnl parameter \u2013 detectable on future Cosmic Microwave
Background (CMB) high\u2013resolution anisotropy maps \u2013 produced by unresolved ex-
tragalactic point sources at frequencies of cosmological interest (45\u2013375 GHz). We
consider two scenarios: an ideal (noiseless) mission and a possible future space-borne
satellite, with instrumental characteristics similar to the ones proposed for the Cos-
mic Origins Explorer (COrE). The local, equilateral, orthogonal and flat shapes are
considered in both temperature (intensity) and polarized emission data. The angular
power spectrum and bispectrum of extragalactic point sources are estimated by state-
of-the-art models of source number counts. The impact of all the most relevant (far\u2013IR
and radio selected) source populations on these shapes at COrE frequencies is studied.
The results of this analysis show that unresolved extragalactic point sources should
not induce a very relevant non-Gaussian signal in the frequency range 100\u2013200GHz,
thus not preventing a correct estimate of the CMB primordial fnl parameter. Polariza-
tion information allows one to significantly reduce the error\u2013bars in the fnl parameter
and the bias induced by unresolved sources and, hence, to widen the range of frequen-
cies for fnl studies. On the contrary, at \u3bd 225GHz, important
non-Gaussian deviations in CMB anisotropy maps are expected due to unresolved
extragalactic sources
The extragalactic radio-source population at 95 GHz
We have used the Australia Telescope Compact Array (ATCA) at 95GHz to carry
out continuum observations of 130 extragalactic radio sources selected from the
Australia Telescope 20GHz (AT20G) survey. Over 90% of these sources are
detected at 95 GHz, and we use a triple-correlation method to measure
simultaneous 20 and 95 GHz flux densities. We show that the ATCA can measure
95GHz flux densities to ~10% accuracy in a few minutes for sources above
~50mJy.
The median 20-95GHz spectral index does not vary significantly with flux
density for extragalactic sources with S20>150 mJy. This allows us to estimate
the extragalactic radio source counts at 95GHz by combining our observed
20-95GHz spectral-index distribution with the accurate 20GHz source counts
measured in the AT20G survey. The resulting 95GHz source counts down to 80 mJy
are significantly lower than those found by several previous studies. The main
reason is that most radio sources with flat or rising spectra in the frequency
range 5-20GHz show a spectral turnover between 20 and 95 GHz. As a result,
there are fewer 95GHz sources (by almost a factor of two at 0.1 Jy) than would
be predicted on the basis of extrapolation from the source populations seen in
lower-frequency surveys. We also derive the predicted confusion noise in CMB
surveys at 95GHz and find a value 20-30% lower than previous estimates.
The 95GHz source population at the flux levels probed by this study is
dominated by QSOs with a median redshift z~1. We find a correlation between
optical magnitude and 95GHz flux density which suggests that many of the
brightest 95 GHz sources are relativistically beamed, with both the optical and
millimetre continuum significantly brightened by Doppler boosting.Comment: Replaced with final version (MNRAS, in press), 15 pages plus two
landscape data table
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