arXiv subjects

Pablo Fosalba

Publications and source records attributed to Pablo Fosalba.

62 records · Page 4Linked to original sources

arXiv2004

The Angular Power Spectrum of the First-Year WMAP Data Reanalysed

We measure the angular power spectrum of the WMAP first-year temperature anisotropy maps. We use SpICE (Spatially Inhomogeneous Correlation Estimator) to estimate Cl's for multipoles l=2-900 from all possible cross-correlation channels. Except for the map-making stage, our measurements provide an independent analysis of that by Hinshaw etal (2003). Despite the different methods used, there is virtually no difference between the two measurements for l < 700 ; the highest l's are still compatible within 1-sigma errors. We use a novel intra-bin variance method to constrain Cl errors in a model independent way. When applied to WMAP data, the intra-bin variance estimator yields diagonal errors 10% larger than those reported by the WMAP team for 100 < l < 450. This translates into a 2.4 sigma detection of systematics since no difference is expected between the SpICE and the WMAP team estimator window functions in this multipole range. With our measurement of the Cl's and errors, we get chi^2/d.o.f. = 1.042 for a best-fit LCDM model, which has a 14% probability, whereas the WMAP team obtained chi^2/d.o.f. = 1.066, which has a 5% probability. We assess the impact of our results on cosmological parameters using Markov Chain Monte Carlo simulations. From WMAP data alone, assuming spatially flat power law LCDM models, we obtain the reionization optical depth tau = 0.145 +/- 0.067, spectral index n_s = 0.99 +/- 0.04, Hubble constant h = 0.67 +/- 0.05, baryon density Omega_b h^2 = 0.0218 +/- 0.0014, cold dark matter density Omega_{cdm} h^2 = 0.122 +/- 0.018, and sigma_8 = 0.92 +/- 0.12, consistent with a reionization redshift z_{re} = 16 +/- 5 (68% CL).

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arXiv2004

Measurement of the gravitational potential evolution from the cross-correlation between WMAP and the APM Galaxy survey

Models with late time cosmic acceleration, such as the Lambda-dominated CDM model, predict a freeze out for the growth of linear gravitational potential at moderate redshift z<1, what can be observed as temperature anisotropies in the CMB: the so called integrated Sachs-Wolfe (ISW) effect. We present a direct measurement of the ISW effect based on the angular cross-correlation function, w_{TG}, of CMB temperature anisotropies and dark-matter fluctuations traced by galaxies. We cross-correlate the first-year WMAP data in combination with the APM Galaxy survey. On the largest scales, theta = 4-10 deg, we detect a non-vanishing cross-correlation at 98.8 % significance level, with a 1-sigma error of w_{TG} = 0.35 +/- 0.14 microK, which favors large values of Omega_Lambda \simeq 0.8 for flat FRW models. On smaller scales, theta < 1deg, the correlations disappear. This is contrary to what would be expected from the ISW effect, but the absence of correlations may be simply explained if the ISW signal was being cancelled by anti-correlations arising from the thermal Sunyaev-Zeldovich (SZ) effect.

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arXiv2003

Cross-correlating the Microwave Sky with Galaxy Surveys

We present results for the cross-correlation between the WMAP 1st-year cosmic microwave background (CMB) anisotropy data and optical galaxy surveys: the APM and SDSS DR1 catalogs. Our measurement of a positive CMB-galaxy correlation on large angles (θ> 4 deg) yields significant detections of the Integrated Sachs-Wolfe (ISW) effect and provides a new estimate of dark-energy in the universe, Ω_Λ=0.69-0.86 (2 σrange). In addition, the correlated signal on small angles (θ<1 deg) reveals the imprint left by hot intra-cluster gas in the CMB photons: the thermal Sunyaev-Zeldovich (SZ) effect

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arXiv2003

Detection of the ISW and SZ effects from the CMB-Galaxy correlation

We present a cross-correlation analysis of the WMAP cosmic microwave background (CMB) temperature anisotropies and the SDSS galaxy density fluctuations. We find significant detections of the angular CMB-galaxy correlation for both the flux limited galaxy sample (z~0.3) and the high redshift (z ~ 0.5) color selected sample. The signal is compatible with that expected from the integrated Sachs-Wolfe (ISW) effect at large angles (θ> 3deg) and the Sunyaev-Zeldovich (SZ) effect at small scales (θ< 1 deg). The detected correlation at low-z is in good agreement with a previous analysis using the APM survey (z~0.15). The combined analysis of all 3 samples yields a total significance better than 3 sigma for ISW and about 2.7 σfor SZ, with a Compton parameter y~10^(-6). For a given flat LCDM model, the ISW effect depends both on the value of Ω_Λand the galaxy bias b. To break this degeneracy, we estimate the bias using the ratio between the galaxy and mass auto-correlation functions in each sample. With our bias estimation, all samples consistently favor a best fit dark-energy dominated model: Ω_Λ~ 0.8, with a 2 σerror Ω_Λ=0.69-0.86.

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arXiv2001

Dust Polarization From Starlight Data

We present a statistical analysis of the interstellar medium (ISM) polarization from the largest compilation available of starlight data, which comprises ~ 5500 stars. The measured correlation between the mean polarization degree and extinction indicates that ISM dust grains are not fully aligned with the uniform component of the large-scale Galactic magnetic field. Moreover, we estimate the ratio of the uniform to the random plane-of-the-sky components of the magnetic field to be B_u/B_r = 0.8. From the analysis of starlight polarization degree and position angle we find that the magnetic field broadly follows Galactic structures on large-scales. On the other hand, the angular power spectrum C_l of the polarization degree for Galactic plane data is found to be consistent with a power-law, C_l ~ l^{-1.5} (where l = 180 deg/θis the multipole order), for angular scales θ> 10 arcmin. We argue that this data set can be used to estimate diffuse polarized emission at microwave frequencies.

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arXiv2001

Statistical Properties of Galactic Starlight Polarization

We present a statistical analysis of Galactic interstellar polarization from the largest compilation available of starlight data. The data comprises ~ 9300 stars of which we have selected ~ 5500 for our analysis. We find a nearly linear growth of mean polarization degree with extinction. The amplitude of this correlation shows that interstellar grains are not fully aligned with the Galactic magnetic field, which can be interpreted as the effect of a large random component of the field. In agreement with earlier studies of more limited scope, we estimate the ratio of the uniform to the random plane-of-the-sky components of the magnetic field to be B_u/B_r = 0.8. Moreover, a clear correlation exists between polarization degree and polarization angle what provides evidence that the magnetic field geometry follows Galactic structures on large-scales. The angular power spectrum C_l of the starlight polarization degree for Galactic plane data (|b| < 10 deg) is consistent with a power-law, C_l ~ l^{-1.5} (where l ~ 180 deg/θis the multipole order), for all angular scales θ> 10 arcmin. An investigation of sparse and inhomogeneous sampling of the data shows that the starlight data analyzed traces an underlying polarized continuum that has the same power spectrum slope, C_l ~ l^{-1.5}. Our findings suggest that starlight data can be safely used for the modeling of Galactic polarized continuum emission at other wavelengths.

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arXiv2000

All-sky convolution for polarimetry experiments

We discuss all-sky convolution of the instrument beam with the sky signal in polarimetry experiments, such as the Planck mission which will map the temperature anisotropy and polarization of the cosmic microwave background (CMB). To account properly for stray light (from e.g. the galaxy, sun, and planets) in the far side-lobes of such an experiment, it is necessary to perform the beam convolution over the full sky. We discuss this process in multipole space for an arbitrary beam response, fully including the effects of beam asymmetry and cross-polarization. The form of the convolution in multipole space is such that the Wandelt-Gorski fast technique for all-sky convolution of scalar signals (e.g. temperature) can be applied with little modification. We further show that for the special case of a pure co-polarized, axisymmetric beam the effect of the convolution can be described by spin-weighted window functions. In the limits of a small angle beam and large Legendre multipoles, the spin-weight 2 window function for the linear polarization reduces to the usual scalar window function used in previous analyses of beam effects in CMB polarimetry experiments. While we focus on the example of polarimetry experiments in the context of CMB studies, we emphasise that the formalism we develop is applicable to anisotropic filtering of arbitrary tensor fields on the sphere.

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arXiv1998

Cosmological Perturbation Theory and the Spherical Collapse Model - II. Non-Gaussian initial conditions

In Part I of this series, we introduced the Spherical Collapse (SC) approximation in Lagrangian space as a way of estimating the cumulants $ξ_J$ of density fluctuations in cosmological Perturbation Theory (PT). Within this approximation, the dynamics is decoupled from the statistics of the initial conditions, so we are able to present here the cumulants for generic Non-Gaussian initial conditions, which can be estimated to arbitrary order including the smoothing effects. The SC model turns out to recover the exact leading-order non-linear contributions up to terms involving non-local integrals of the $J$-point functions. We argue that for the hierarchical ratios $S_J$, these non-local terms are sub-dominant and tend to compensate each other. The resulting predictions show a non-trivial time evolution that can be used to discriminate between models of structure formation. We compare these analytic results to Non-Gaussian N-body simulations, which turn out to be in very good agreement up to scales where $σ\simlt 1$.

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