462 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
Unravelling mechanistic insights in the platinum-catalysed dihydroalkoxylation of allenes
The mechanism of the platinum-catalysed dihydroalkoxylation of allenes to give acetals has been studied experimentally and by computational methods. Our findings further explain divergent reactivity encountered for platinum- and gold-vinyl intermediates after the first nucleophilic attack onto the coordinated allene, as well as provide new details on the catalytic cycle with platinum, uncovering enol ethers as resting states of the catalytic cycle, a S(E)Ox process via Pt(V)-H as the final protodemetallation step after the second nucleophilic attack when neutral platinum complexes are used, and a fast acid promoted addition of methanol to enol ethers when cationic platinum complexes are employed
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
Early evolution of galaxies and of large-scale structure from CMB experiments
Next generation CMB experiments with arcmin resolution will, for free, lay
the foundations for a real breakthrough on the study of the early evolution of
galaxies and galaxy clusters, thanks to the detection of large samples of
strongly gravitationally lensed galaxies and of proto-clusters of dusty
galaxies up to high redshifts. This has an enormous legacy value. High
resolution follow-up of strongly lensed galaxies will allow the direct
investigation of their structure and kinematics up to z~6, providing direct
information on physical processes driving their evolution. Follow-up of
proto-clusters will allow an observational validation of the formation history
of the most massive dark matter halos up to z~4, well beyond the redshift range
accessible via X-ray or SZ measurements. These experiments will also allow a
giant leap forward in the determination of polarization properties of
extragalactic sources, and will provide a complete census of cold dust
available for star formation in the local universe.Comment: Science white paper submitted to the Astro2020 US Decadal Surve
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
Radio sources in next-generation CMB surveys
CMB surveys provide, for free, blindly selected samples of extragalactic
radio sources at much higher frequencies than traditional radio surveys.
Next-generation, ground-based CMB experiments with arcmin resolution at mm
wavelengths will provide samples of thousands radio sources allowing the
investigation of the evolutionary properties of blazar populations, the study
of the earliest and latest stages of radio activity, the discovery of rare
phenomena and of new transient sources and events. Space-borne experiments will
extend to sub-mm wavelengths the determinations of the SEDs of many hundreds of
blazars, in temperature and in polarization, allowing us to investigate the
flow and the structure of relativistic jets close to their base, and the
electron acceleration mechanisms. A real breakthrough will be achieved in the
caracterization of the polarization properties. The first direct counts in
polarization will be obtained, enabling a solid assessment of the
extra-galactic source contamination of CMB maps and allowing us to understand
structure and intensity of magnetic fields, particle densities and structures
of emitting regions close to the base of the jet.Comment: Science white paper submitted to the Astro2020 US Decadal Surve
- …