arXiv subjects

J. Bel

Publications and source records attributed to J. Bel.

58 records · Page 4Linked to original sources

arXiv2012

The Scale of Cosmic Isotropy

The most fundamental premise to the standard model of the universe, the Cosmological Principle (CP), states that the large-scale properties of the universe are the same in all directions and at all comoving positions. Demonstrating this theoretical hypothesis has proven to be a formidable challenge. The cross-over scale R_{iso} above which the galaxy distribution becomes statistically isotropic is vaguely defined and poorly (if not at all) quantified. Here we report on a formalism that allows us to provide an unambiguous operational definition and an estimate of R_{iso}. We apply the method to galaxies in the Sloan Digital Sky Survey (SDSS) Data Release 7, finding that R_{iso}\sim 150h^{-1} Mpc. Besides providing a consistency test of the Copernican principle, this result is in agreement with predictions based on numerical simulations of the spatial distribution of galaxies in cold dark matter dominated cosmological models.

astro-ph.CO↗

arXiv2012

Second-order matter fluctuations via higher-order galaxy correlators

We provide a formula for extracting the value of the rms of the linear matter fluctuations on a scale R directly from redshift surveys data. It allows to constrain the real-space amplitude of sigma_R without requiring any modeling of the nature and power spectrum of the matter distribution. Furthermore, the formalism is completely insensitive to the character of the bias function, namely its eventual scale or non-linear dependence. By contrasting measurements of sigma_R with predictions from linear perturbation theory, one can test for eventual departures from the standard description of gravity on large cosmological scales. The proposed estimator exploits the information contained in the 1-point moments and 2-point correlators of the matter and galaxy density fields, and it can be applied on cosmic scales where linear and semi-linear perturbative approximations of the evolution of matter overdensities offer a satisfactory description of the full underlying theory. We implement the test with N-body simulations to quantify potential systematics and successfully show that we are able to recover the present day value of sigma_8 `hidden' in the simulation. We also design a consistency check to gauge the soundness of the results inferred when the formalism is applied to real (as opposed to simulated) data. We expect that this approach will provide a sensitive probe of the clustering of matter when applied to future large redshift survey such as BigBOSS and EUCLID.

astro-ph.CO↗

arXiv2012

The VIMOS Public Extragalactic Redshift Survey (VIPERS): spectral classification through Principal Component Analysis

We develop a Principal Component Analysis aimed at classifying a sub-set of 27,350 spectra of galaxies in the range 0.4 < z < 1.0 collected by the VIMOS Public Extragalactic Redshift Survey (VIPERS). We apply an iterative algorithm to simultaneously repair parts of spectra affected by noise and/or sky residuals, and reconstruct gaps due to rest-frame transformation, and obtain a set of orthogonal spectral templates that span the diversity of galaxy types. By taking the three most significant components, we find that we can describe the whole sample without contamination from noise. We produce a catalogue of eigen-coefficients and template spectra that will be part of future VIPERS data releases. Our templates effectively condense the spectral information into two coefficients that can be related to the age and star formation rate of the galaxies. We examine the spectrophotometric types in this space and identify early, intermediate, late and starburst galaxies.

astro-ph.CO↗

arXiv2012

The power spectrum from the angular distribution of galaxies in the CFHTLS-Wide fields at redshift ~0.7

We measure the real-space galaxy power spectrum on large scales at redshifts 0.5 to 1.2 using optical colour-selected samples from the CFHT Legacy Survey. With the redshift distributions measured with a preliminary ~14000 spectroscopic redshifts from the VIMOS Public Extragalactic Redshift Survey (VIPERS), we deproject the angular distribution and directly estimate the three-dimensional power spectrum. We use a maximum likelihood estimator that is optimal for a Gaussian random field giving well-defined window functions and error estimates. This measurement presents an initial look at the large-scale structure field probed by the VIPERS survey. We measure the galaxy bias of the VIPERS-like sample to be b_g=1.38 +- 0.05 (sigma_8=0.8) on scales k<0.2h/mpc averaged over 0.5<z<1.2. We further investigate three photometric redshift slices, and marginalising over the bias factors while keeping other LCDM parameters fixed, we find the matter density Omega_m=0.30+-0.06.

astro-ph.CO↗