782 research outputs found
Detection/estimation of the modulus of a vector. Application to point source detection in polarization data
Given a set of images, whose pixel values can be considered as the components
of a vector, it is interesting to estimate the modulus of such a vector in some
localised areas corresponding to a compact signal. For instance, the
detection/estimation of a polarized signal in compact sources immersed in a
background is relevant in some fields like astrophysics. We develop two
different techniques, one based on the Neyman-Pearson lemma, the Neyman-Pearson
filter (NPF), and another based on prefiltering-before-fusion, the filtered
fusion (FF), to deal with the problem of detection of the source and estimation
of the polarization given two or three images corresponding to the different
components of polarization (two for linear polarization, three including
circular polarization). For the case of linear polarization, we have performed
numerical simulations on two-dimensional patches to test these filters
following two different approaches (a blind and a non-blind detection),
considering extragalactic point sources immersed in cosmic microwave background
(CMB) and non-stationary noise with the conditions of the 70 GHz \emph{Planck}
channel. The FF outperforms the NPF, especially for low fluxes. We can detect
with the FF extragalactic sources in a high noise zone with fluxes >=
(0.42,0.36) Jy for (blind/non-blind) detection and in a low noise zone with
fluxes >= (0.22,0.18) Jy for (blind/non-blind) detection with low errors in the
estimated flux and position.Comment: 11 pages, 5 figure
A multifrequency method based on the Matched Multifilter for the detection of point sources in CMB maps
In this work we deal with the problem of simultaneous multifrequency
detection of extragalactic point sources in maps of the Cosmic Microwave
Background. We apply a linear filtering technique that uses spatial information
and the cross-power spectrum. To make this, we simulate realistic and
non-realistic flat patches of the sky at two frequencies of Planck: 44 and 100
GHz. We filter to detect and estimate the point sources and compare this
technique with the monofrequency matched filter in terms of completeness,
reliability, flux and spectral index accuracy. The multifrequency method
outperforms the matched filter at the two frequencies and in all the studied
cases in the work.Comment: 14 pages, 6 figures, 1 tabl
A fast Bayesian approach to discrete object detection in astronomical datasets - PowellSnakes I
A new fast Bayesian approach is introduced for the detection of discrete
objects immersed in a diffuse background. This new method, called PowellSnakes,
speeds up traditional Bayesian techniques by: i) replacing the standard form of
the likelihood for the parameters characterizing the discrete objects by an
alternative exact form that is much quicker to evaluate; ii) using a
simultaneous multiple minimization code based on Powell's direction set
algorithm to locate rapidly the local maxima in the posterior; and iii)
deciding whether each located posterior peak corresponds to a real object by
performing a Bayesian model selection using an approximate evidence value based
on a local Gaussian approximation to the peak. The construction of this
Gaussian approximation also provides the covariance matrix of the uncertainties
in the derived parameter values for the object in question. This new approach
provides a speed up in performance by a factor of `hundreds' as compared to
existing Bayesian source extraction methods that use MCMC to explore the
parameter space, such as that presented by Hobson & McLachlan. We illustrate
the capabilities of the method by applying to some simplified toy models.
Furthermore PowellSnakes has the advantage of consistently defining the
threshold for acceptance/rejection based on priors which cannot be said of the
frequentist methods. We present here the first implementation of this technique
(Version-I). Further improvements to this implementation are currently under
investigation and will be published shortly. The application of the method to
realistic simulated Planck observations will be presented in a forthcoming
publication.Comment: 30 pages, 15 figures, revised version with minor changes, accepted
for publication in MNRA
Herschel-ATLAS: Evolution of the 250 µm luminosity function out to z = 0.5
5 páginas, 4 figuras.-- Herschel is an ESA space observatory with science instruments
provided by European-led Principal Investigator consortia and with important
participation from NASA.-- et al.We have determined the luminosity function of 250 μm-selected galaxies detected in the ~14 deg2 science demonstration region of the Herschel-ATLAS project out to a redshift of z = 0.5. Our findings very clearly show that the luminosity function evolves steadily out to this redshift. By selecting a sub-group of sources within a fixed luminosity interval where incompleteness effects are minimal, we have measured a smooth increase in the comoving 250 μm luminosity density out to z = 0.2 where it is 3.6+1.4-0.9 times higher than the local value.S.D. Acknowledges the UK STFC for support.Peer reviewe
The estimation of the SZ effects with unbiased multifilters
In this work we study the performance of linear multifilters for the
estimation of the amplitudes of the thermal and kinematic Sunyaev-Zel'dovich
effects. We show that when both effects are present, estimation of these
effects with standard matched multifilters is intrinsically biased. This bias
is due to the fact that both signals have basically the same spatial profile.
We find a new family of multifilters related to the matched multifilters that
cancel this systematic bias, hence we call them Unbiased Matched Multifilters.
We test the unbiased matched multifilters and compare them with the standard
matched multifilters using simulations that reproduce the future Planck
mission's observations. We find that in the case of the standard matched
multifilters the systematic bias in the estimation of the kinematic
Sunyaev-Zel'dovich effect can be very large, even greater than the statistical
error bars. Unbiased matched multifilters cancel effectively this kind of bias.
In concordance with other works in the literature, our results indicate that
the sensitivity and resolution of Planck will not be enough to give reliable
estimations of the kinematic Sunyaev-Zel'dovich of individual clusters.
However, since the estimation with the unbiased matched multifilters is not
intrinsically biased, it can be possible to use them to statistically study
peculiar velocities in large scales using large sets of clusters.Comment: 12 pages, 6 figures, submitted to MNRA
Transport of cattle in Spain. Technical, administrative and welfare aspects according to the destination.
To obtain a thorough description of how are cattle transported in Spain with respect to the journey destination a survey was performed in 2004 and 2005. Information was obtained by means of a 119-parameters questionnaire, and 44 transport operators representative of the sector (27 slaughterhouses, 10 traders, and 7 cattle markets) were interviewed. Over 80% of journeys transported growing-finishing/finished animals, and about 3% of journeys transported both growing-finishing/finished and reproductive animals. With respect to farm transports, slaughter transports loaded in fewer farms (1.2 vs. 1.4 farms; pPara conocer en detalle como se transporta el ganado vacuno en España según el destino del viaje, entre los años 2004 y 2005 se entrevistaron 44 operadores representativos del sector (27 mataderos, 10 comerciantes y 7 mercados) mediante cuestionarios que recogían información relativa a 119 parámetros. Más del 80% de los viajes fueron de animales cebados/para cebo, y algo más de un 3% de los viajes transportaban animales cebados/para cebo y reproductores. Los transportes a matadero cargaron en un menor número de explotaciones (1,2 vs. 1,4 granjas;
Polarization of the WMAP Point Sources
The detection of polarized sources in the WMAP 5-year data is a very
difficult task. The maps are dominated by instrumental noise and only a handful
of sources show up as clear peaks in the Q and U maps. Optimal linear filters
applied at the position of known bright sources detect with a high level of
significance a polarized flux P from many more sources, but estimates of P are
liable to biases. Using a new technique, named the "filtered fusion technique",
we have detected in polarization, with a significance level greater than 99.99%
in at least one WMAP channel, 22 objects, 5 of which, however, do not have a
plausible low radio frequency counterpart and are therefore doubtful. Estimated
polarized fluxes P < 400 mJy at 23 GHz were found to be severely affected by
the Eddington bias. The corresponding polarized flux limit for Planck/LFI at 30
GHz, obtained via realistic simulations, is 300 mJy. We have also obtained
statistical estimates of, or upper limits to the mean polarization degrees of
bright WMAP sources at 23, 33, 41, and 61 GHz, finding that they are of a few
percent.Comment: 10 pages, 6 figures. Accepted for publication in Ap
A novel multi-frequency technique for the detection of point sources in Cosmic Microwave Background maps
In this work we address the problem of simultaneous multi-frequency detection
of extragalactic point sources in maps of the Cosmic Microwave Background. We
apply a new linear filtering technique, the so called `matched matrix filters',
that incorporates full spatial information, including the cross-correlation
among channels, without making any a priori assumption about the spectral
behaviour of the sources. A substantial reduction of the background is achieved
thanks to the optimal combination of filtered maps. We describe in detail the
new technique and we apply it to the detection/estimation of radio sources in
realistic all-sky Planck simulations at 30, 44, 70 and 100 GHz. Then we compare
the results with the mono-frequential approach based on the standard matched
filter, in terms of reliability, completeness and flux accuracy of the
resulting point source catalogs. The new filters outperform the standard
matched filters for all these indexes at 30, 44 and 70 GHz, whereas at 100 GHz
both kind of filters have a similar performance. We find a noticeable increment
of the number of true detections for a fixed reliability level. In particular,
for a 95% reliability we practically double the number of detections at 30, 44
and 70 GHz.Comment: 12 pages, 10 figures, submitted to MNRA
A comparison of algorithms for the construction of SZ cluster catalogues
J.-B. Melin et al.We evaluate the construction methodology of an all-sky catalogue of galaxy clusters detected through the Sunyaev-Zel'dovich (SZ) effect. We perform an extensive comparison of twelve algorithms applied to the same detailed simulations of the millimeter and submillimeter sky based on a Planck-like case. We present the results of this >SZ Challenge> in terms of catalogue completeness, purity, astrometric and photometric reconstruction. Our results provide a comparison of a representative sample of SZ detection algorithms and highlight important issues in their application. In our study case, we show that the exact expected number of clusters remains uncertain (about a thousand cluster candidates at |b| > 20 deg with 90% purity) and that it depends on the SZ model and on the detailed sky simulations, and on algorithmic implementation of the detection methods. We also estimate the astrometric precision of the cluster candidates which is found of the order of ~2 arcmin on average, and the photometric uncertainty of about 30%, depending on flux. © ESO, 2012.Peer Reviewe
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