arXiv subjects

P. Vielva

Publications and source records attributed to P. Vielva.

At least 73 records · Page 4Linked to original sources

arXiv2012

Biparametric Adaptive Filter: detection of compact sources in complex microwave backgrounds

In this article we consider the detection of compact sources in maps of the Cosmic Microwave Background radiation (CMB) following the philosophy behind the Mexican Hat Wavelet Family (MHWn) of linear filters. We present a new analytical filter, the Biparametric Adaptive Filter (BAF), that is able to adapt itself to the statistical properties of the background as well as to the profile of the compact sources, maximizing the amplification and improving the detection process. We have tested the performance of this filter using realistic simulations of the microwave sky between 30 and 857 GHz as observed by the Planck satellite, where complex backgrounds can be found. We demonstrate that doing a local analysis on flat patches allows one to find a combination of the optimal scale of the filter R and the index of the filter g that will produce a global maximum in the amplification, enhancing the signal-to-noise ratio (SNR) of the detected sources in the filtered map and improving the total number of detections above a threshold. We conclude that the new filter is able to improve the overall performance of the MHW2, increasing the SNR of the detections and, therefore, the number of detections above a 5 sigma threshold. The improvement of the new filter in terms of SNR is particularly important in the vicinity of the galactic plane and in the presence of strong galactic emission. Finally, we compare the sources detected by each method and find that the new filter is able to detect more new sources than the MHW2 at all frequencies and in clean regions of the sky. The BAF is also less affected by spurious detections, associated to compact structures in the vicinity of the galactic plane.

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arXiv2011

Multi-resolution internal template cleaning: An application to the Wilkinson Microwave Anisotropy Probe 7-yr polarization data

Cosmic microwave background (CMB) radiation data obtained by different experiments contain, besides the desired signal, a superposition of microwave sky contributions. We present a fast and robust method, using a wavelet decomposition on the sphere, to recover the CMB signal from microwave maps. An application to \textit{WMAP} polarization data is presented, showing its good performance particularly in very polluted regions of the sky. The applied wavelet has the advantages of requiring little computational time in its calculations, being adapted to the \textit{HEALPix} pixelization scheme, and offering the possibility of multi-resolution analysis. The decomposition is implemented as part of a fully internal template fitting method, minimizing the variance of the resulting map at each scale. Using a $χ^2$ characterization of the noise, we find that the residuals of the cleaned maps are compatible with those expected from the instrumental noise. The maps are also comparable to those obtained from the \textit{WMAP} team, but in our case we do not make use of external data sets. In addition, at low resolution, our cleaned maps present a lower level of noise. The E-mode power spectrum $C_{\ell}^{EE}$ is computed at high and low resolution; and a cross power spectrum $C_{\ell}^{TE}$ is also calculated from the foreground reduced maps of temperature given by \textit{WMAP} and our cleaned maps of polarization at high resolution. These spectra are consistent with the power spectra supplied by the \textit{WMAP} team. We detect the E-mode acoustic peak at $\ell \sim 400$, as predicted by the standard $ΛCDM$ model. The B-mode power spectrum $C_{\ell}^{BB}$ is compatible with zero.

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arXiv2011

Wilkinson Microwave Anisotropy Probe 7-yr constraints on fNL with a fast wavelet estimator

A new method to constrain the local non-linear coupling parameter fNL based on a fast wavelet decomposition is presented. Using a multiresolution wavelet adapted to the HEALPix pixelization, we have developed a method that is 10^2 times faster than previous estimators based on isotropic wavelets and 10^3 faster than the KSW bispectrum estimator, at the resolution of the Wilkinson Microwave Anisotropy Probe (WMAP) data. The method has been applied to the WMAP 7-yr V+W combined map, imposing constraints on fNL of -69 < fNL < 65 at the 95 per cent CL. This result has been obtained after correcting for the contribution of the residual point sources which has been estimated to be fNL = 7 +/- 6. In addition, a Gaussianity analysis of the data has been carried out using the third order moments of the wavelet coefficients, finding consistency with Gaussianity. Although the constrainsts imposed on fNL are less stringent than those found with optimal estimators, we believe that a very fast method, as the one proposed in this work, can be very useful, especially bearing in mind the large amount of data that will be provided by future experiments, such as the Planck satellite. Moreover, the localisation of wavelets allows one to carry out analyses on different regions of the sky. As an application, we have separately analysed the two hemispheres defined by the dipolar modulation proposed by Hoftuft et al. (2009). We do not find any significant asymmetry regarding the estimated value of fNL in those hemispheres.

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arXiv2011

An introduction to compact source detection in cosmic microwave background images

We aim to present a tutorial on the detection, parameter estimation and statistical analysis of compact sources (far galaxies, galaxy clusters and Galactic dense emission regions) in cosmic microwave background observations. The topic is of great relevance for current and future cosmic microwave background missions because the presence of compact sources in the data introduces very significant biases in the determination of the cosmological parameters that determine the energy contain, origin and evolution of the universe and because compact sources themselves provide us with important information about the large scale structure of the universe.

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arXiv2011

Cosmic Microwave Background Images

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arXiv2010

Anomalous variance in the WMAP data and Galactic Foreground residuals

A previous work (Monteserín 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.

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arXiv2010

A comprehensive overview of the Cold Spot

The report of a significant deviation of the CMB temperature anisotropies distribution from Gaussianity (soon after the public release of the WMAP data in 2003) has become one of the most solid WMAP anomalies. This detection grounds on an excess of the kurtosis of the Spherical Mexican Hat Wavelet coefficients at scales of around 10 degrees. At these scales, a prominent feature --located in the southern Galactic hemisphere-- was highlighted from the rest of the SMHW coefficients: the Cold Spot. This article presents a comprehensive overview related to the study of the Cold Spot, paying attention to the non-Gaussianity detection methods, the morphological characteristics of the Cold Spot, and the possible sources studied in the literature to explain its nature. Special emphasis is made on the Cold Spot compatibility with a cosmic texture, commenting on future tests that would help to give support or discard this hypothesis.

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arXiv2010

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.

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arXiv2010

Constraints on f_nl and g_nl from the analysis of the N-pdf of the CMB large scale anisotropies

[Abridged] In this paper we explore a local non-linear perturbative model up to third order as a general characterization of the CMB anisotropies. We focus our analysis in large scale anisotropies. At these angular scales, the non-Gaussian description proposed in this work defaults (under certain conditions) to an approximated local form of the weak non-linear coupling inflationary model. In particular, quadratic and cubic terms are governed by the non-linear coupling parameters f_nl and g_nl, respectively. The extension proposed in this paper allows us to directly constrain these non-linear parameters. Applying the proposed methodology to WMAP 5-yr data, we obtain -5.6 x 10^5 < g_nl < 6.4 x 10^5, at 95% CL. This result is in agreement with previous findings obtained for equivalent non-Gaussian models and with different non-Gaussian estimators. A model selection test is performed, indicating that a Gaussian model is preferred to the non-Gaussian scenario. When comparing different non-Gaussian models, we observe that a pure f_nl model is the most favoured case, and that a pure g_nl model is more likely than a general non-Gaussian scenario. Finally, we have analyzed the WMAP data in two independent hemispheres, in particular the ones defined by the dipolar pattern found by Hoftuft et al. 2009. We show that, whereas g_nl is still compatible with zero for both hemispheres, it is not the case for f_nl (with a p-value 0.04). However, if anisotropy of the data is assumed, the distance between the likelihood distributions for each hemisphere is larger than expected from Gaussian and anisotropic simulations, also for g_nl (with a p-value of 0.001 in the case of this parameter). This result is an extra evidence for the CMB asymmetries previously reported in WMAP data.

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arXiv2009

Analysis of non-Gaussian CMB maps based on the N-pdf. Application to WMAP data

We present a new method based on the N-point probability distribution (pdf) to study non-Gaussianity in cosmic microwave background (CMB) maps. Likelihood and Bayesian estimation are applied to a local non-linear perturbed model up to third order, characterized by a linear term which is described by a Gaussian N-pdf, and a second and third order terms which are proportional to the square and the cube of the linear one. We also explore a set of model selection techniques (the Akaike and the Bayesian Information Criteria, the minimum description length, the Bayesian Evidence and the Generalized Likelihood Ratio Test) and their application to decide whether a given data set is better described by the proposed local non-Gaussian model, rather than by the standard Gaussian temperature distribution. As an application, we consider the analysis of the WMAP 5-year data at a resolution of around 2 degrees. At this angular scale (the Sachs-Wolfe regime), the non-Gaussian description proposed in this work defaults (under certain conditions) to an approximative local form of the weak non-linear coupling inflationary model (e.g. Komatsu & Spergel 2001) previously addressed in the literature. For this particular case, we obtain an estimation for the non-linear coupling parameter of -94 < F_nl < 154 at 95% CL. Equally, model selection criteria also indicate that the Gaussian hypothesis is favored against the particular local non-Gaussian model proposed in this work. This result is in agreement with previous findings obtained for equivalent non-Gaussian models and with different non-Gaussian estimators. However, our estimator based on the N-pdf is more efficient than previous estimators and, therefore, provides tighter constraints on the coupling parameter at degree angular resolution.

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arXiv2009

The WMAP cold spot

The WMAP cold spot was found by applying spherical wavelets to the first year WMAP data. An excess of kurtosis of the wavelet coefficient was observed at angular scales of around 5 degrees. This excess was shown to be inconsistent with Gaussian simulations with a p-value of around 1%. A cold spot centered at (b = -57, l = 209) was shown to be the main cause of this deviation. Several hypotheses were raised to explain the origin of the cold spot. After performing a Bayesian template fit a collapsing cosmic texture was found to be the most probable hypothesis explaining the spot. Here we review the properties of the cold spot and the possible explanations.

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arXiv2008

Component separation methods for the Planck mission

The Planck satellite will map the full sky at nine frequencies from 30 to 857 GHz. The CMB intensity and polarization that are its prime targets are contaminated by foreground emission. The goal of this paper is to compare proposed methods for separating CMB from foregrounds based on their different spectral and spatial characteristics, and to separate the foregrounds into components of different physical origin. A component separation challenge has been organized, based on a set of realistically complex simulations of sky emission. Several methods including those based on internal template subtraction, maximum entropy method, parametric method, spatial and harmonic cross correlation methods, and independent component analysis have been tested. Different methods proved to be effective in cleaning the CMB maps from foreground contamination, in reconstructing maps of diffuse Galactic emissions, and in detecting point sources and thermal Sunyaev-Zeldovich signals. The power spectrum of the residuals is, on the largest scales, four orders of magnitude lower than that of the input Galaxy power spectrum at the foreground minimum. The CMB power spectrum was accurately recovered up to the sixth acoustic peak. The point source detection limit reaches 100 mJy, and about 2300 clusters are detected via the thermal SZ effect on two thirds of the sky. We have found that no single method performs best for all scientific objectives. We foresee that the final component separation pipeline for Planck will involve a combination of methods and iterations between processing steps targeted at different objectives such as diffuse component separation, spectral estimation and compact source extraction.

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arXiv2008

The Quijote CMB Experiment

We present the current status of the QUIJOTE (Q-U-I JOint TEnerife) CMB Experiment, a new instrument which will start operations early 2009 at Teide Observatory, with the aim of characterizing the polarization of the CMB and other processes of galactic and extragalactic emission in the frequency range 10-30 GHz and at large angular scales. QUIJOTE will be a valuable complement at low frequencies for the PLANCK mission, and will have the required sensitivity to detect a primordial gravitational-wave component if the tensor-to-scalar ratio is larger than r=0.05.

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arXiv2008

The CMB cold spot: texture, cluster or void?

The non-Gaussian cold spot found in the WMAP data has created controversy about its origin. Here we calculate the Bayesian posterior probability ratios for three different models that could explain the cold spot. A recent work claimed that the Spot could be caused by a cosmic texture, while other papers suggest that it could be due to the gravitational effect produced by an anomalously large void. Also the Sunyaev-Zeldovich effect caused by a cluster is taken into account as a possible origin. We perform a template fitting on a 20 degrees radius patch centered at Galactic coordinates (b = -57circ, l = 209) and calculate the posterior probability ratios for the void and Sunyaev-Zeldovich models, comparing the results to those obtained with textures. Taking realistic priors for the parameters, the texture interpretation is favored, while the void and Sunyaev-Zeldovich hypotheses are discarded. The temperature decrement produced by voids or clusters is negligible considering realistic values for the parameters.

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arXiv2008

A linear filter to reconstruct the ISW effect from CMB and LSS observations

The extraction of a signal from some observational data sets that contain different contaminant emissions, often at a greater level than the signal itself, is a common problem in Astrophysics and Cosmology. The signal can be recovered, for instance, using a simple Wiener filter. However, in certain cases, additional information may also be available, such as a second observation which correlates to a certain level with the sought signal. In order to improve the quality of the reconstruction, it would be useful to include as well this additional information. Under these circumstances, we have constructed a linear filter, the linear covariance-based filter, that extracts the signal from the data but takes also into account the correlation with the second observation. To illustrate the performance of the method, we present a simple application to reconstruct the so-called Integrated Sachs-Wolfe effect from simulated observations of the Cosmic Microwave Background and of catalogues of galaxies.

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arXiv2008

A low CMB variance in the WMAP data

We have estimated the CMB variance from the three-year WMAP data, finding a value which is significantly lower than the one expected from Gaussian simulations using the WMAP best-fit cosmological model, at a significance level of 98.7 per cent. This result is even more prominent if we consider only the north ecliptic hemisphere (99.8 per cent). Different analyses have been performed in order to identify a possible origin for this anomaly. In particular we have studied the behaviour of single radiometers and single year data as well as the effect of residual foregrounds and 1/f noise, finding that none of these possibilities can explain the low value of the variance. We have also tested the effect of varying the cosmological parameters, finding that the estimated CMB variance tends to favour higher values of $n_s$ than the one of the WMAP best-fit model. In addition, we have also tested the consistency between the estimated CMB variance and the actual measured CMB power spectrum of the WMAP data, finding a strong discrepancy. A possible interpretation of this result could be a deviation from Gaussianity and/or isotropy of the CMB.

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arXiv2008

Non-Gaussianity analysis on local morphological measures of WMAP data

The decomposition of a signal on the sphere with the steerable wavelet constructed from the second Gaussian derivative gives access to the orientation, signed-intensity, and elongation of the signal's local features. In the present work, the non-Gaussianity of the WMAP temperature data of the cosmic microwave background (CMB) is analyzed in terms of the first four moments of the statistically isotropic random fields associated with these local morphological measures, at wavelet scales corresponding to angular sizes between 27.5 arcminutes and 30 degrees on the celestial sphere. While no detection is made neither in the orientation analysis nor in the elongation analysis, a strong detection is made in the excess kurtosis of the signed-intensity of the WMAP data. The non-Gaussianity is observed with a significance level below 0.5% at a wavelet scale corresponding to an angular size around 10 degrees, and confirmed at neighbour scales. This supports a previous detection of an excess of kurtosis in the wavelet coefficient of the WMAP data with the axisymmetric Mexican hat wavelet (Vielva et al. 2004). Instrumental noise and foreground emissions are not likely to be at the origin of the excess of kurtosis. Large-scale modulations of the CMB related to some unknown systematics are rejected as possible origins of the detection. The observed non-Gaussianity may therefore probably be imputed to the CMB itself, thereby questioning the basic inflationary scenario upon which the present concordance cosmological model relies. Taking the CMB temperature angular power spectrum of the concordance cosmological model at face value, further analysis also suggests that this non-Gaussianity is not confined to the directions on the celestial sphere with an anomalous signed-intensity.

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arXiv2007

Bianchi VII_h models and the cold spot texture

We have returned to our previous Bianchi VII_h analysis in light of the Cruz et al. 2007 suggestion that the cold spot observed near the southern Galactic pole may be a remnant temperature perturbation of a cosmic texture. In Bridges et al. 2006b we found two favoured left handed Bianchi VII_h templates with restricted prior probabilities so that the template was centred close to the cold spot. Using WMAP data `corrected' for the texture fit we have now reexamined both models to assess any changes to these conclusions. We find that both models are left almost entirely unconstrained by the data and consequently exhibit significantly reduced Bayesian evidences. Both models are now disfavoured by the data. This result reinforces our previous suggestion that the cold spot was likely to be driving any Bianchi VII_h detection.

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