Alexandre Refregier

Weak Lensing Measurements: A Revisited Method and Application toHubble Space Telescope Images

The weak distortions produced by gravitational lensing in the images of background galaxies provide a unique method to measure directly the distribution of mass in the universe. However, because the induced distortions are only of a few percent, this technique requires high-precision measurements of the lensing shear and cautious corrections for systematic effects. Kaiser, Squires, & Broadhurst proposed a method to calibrate the ellipticity-shear relation in the presence of point-spread function (PSF) anisotropies and camera distortions. Here, we revisit the Kaiser, Squires, & Broadhurst method in the context of the demanding search for weak lensing by large-scale structure. We show that both the PSF and the camera distortions can be corrected for using source moments, as opposed to ellipticities. We clarify the applicability of some of the approximations made in this method. We derive expressions for the corrections that involve only the galaxy moments. By decomposing the moments into spinors, we derive an explicit relation between the shear and the average ellipticity. We discuss the shortcomings of the method and test its validity using numerical simulations. As an application of the method, we repeat the analysis of the Hubble Space Telescope (HST) WFPC2 camera performed by Hoekstra et al. We confirm the presence of sizable (~10%) PSF ellipticities at the edge of the WFPC2 chips. However, we find that the camera distortion is radial, rather than tangential. We also show that the PSF ellipticity varies by as much as 2% over time. We use these measurements to correct the shape of galaxies in the HST Survey Strip (the Groth Strip). By considering the dependence of the ellipticities on object size, we show that, after corrections, the residual systematic uncertainty for galaxies with radii greater than 015 is about 0.4% when averaged over each chip. We discuss how these results provide good prospects for measuring weak lensing by large-scale structure with deep HST surveys.

Weak Lensing Measurements: A Revisited Method and Application to HST Images

The weak distortions produced by gravitational lensing in the images of background galaxies provide a unique method to measure directly the distribution of mass in the universe. However, because the induced distortions are only of a few percent, this technique requires high-precision measurements of the lensing shear and cautious corrections for systematic effects. Kaiser, Squires, & Broadhurst proposed a method to calibrate the ellipticity-shear relation in the presence of point-spread function (PSF) anisotropies and camera distortions. Here, we revisit the Kaiser, Squires, & Broadhurst method in the context of the demanding search for weak lensing by large-scale structure. We show that both the PSF and the camera distortions can be corrected for using source moments, as opposed to ellipticities. We clarify the applicability of some of the approximations made in this method. We derive expressions for the corrections that involve only the galaxy moments. By decomposing the moments into spinors, we derive an explicit relation between the shear and the average ellipticity. We discuss the shortcomings of the method and test its validity using numerical simulations. As an application of the method, we repeat the analysis of the Hubble Space Telescope (HST) WFPC2 camera performed by Hoekstra et al. We confirm the presence of sizable (~10%) PSF ellipticities at the edge of the WFPC2 chips. However, we find that the camera distortion is radial, rather than tangential. We also show that the PSF ellipticity varies by as much as 2% over time. We use these measurements to correct the shape of galaxies in the HST Survey Strip (the Groth Strip). By considering the dependence of the ellipticities on object size, we show that, after corrections, the residual systematic uncertainty for galaxies with radii greater than 015 is about 0.4% when averaged over each chip. We discuss how these results provide good prospects for measuring weak lensing by large-scale structure with deep HST surveys.

Power spectrum of the Sunyaev-Zel'dovich effect

(Abridged) The hot gas in the IGM produces anisotropies in the Cosmic Microwave Background (CMB) through the thermal Sunyaev-Zeldovich (SZ) effect. The SZ effect is a powerful probe of large-scale structure in the universe and must be carefully subtracted from measurements of the primary CMB anisotropies. We use moving-mesh hydrodynamical simulations to study the 3-dimensional statistics of the gas, and compute the mean comptonization parameter and the angular power spectrum of the SZ fluctuations, for different cosmologies. We compare these results with predictions using the Press-Schechter formalism. We find that the two methods agree approximately, but differ in details. We discuss this discrepancy, and show that resolution limits the reliability of our results to the 200<l<2000 range. For cluster- normalized CDM models, the SZ power spectrum is comparable to the primordial power spectrum around l=2000. We show that groups and filaments (kT<5 keV) contribute about 50% of the SZ power spectrum at l=500. About half of the SZ power spectrum on these scales is produced at redshifts z<0.1, and can thus be detected and removed using existing catalogs of galaxies and X-ray clusters. We discuss the implications of these results for the future MAP and Planck Surveyor missions.

THEORY AND STATISTICS OF WEAK LENSING FROM LARGE-SCALE MASS INHOMOGENEITIES

Weak lensing by large-scale mass inhomogeneities in the Universe induces correlations in the observed ellipticities of distant sources. We first review the harmonic analysis and statistics required of these correlations and discuss calculations for the predicted signal. We consider the ellipticity correlation function, the mean-square ellipticity, the ellipticity power spectrum and a global maximum-likelihood analysis to isolate a weak-lensing signal from the data. Estimates for the sensitivity of a survey of a given area, surface density, and mean intrinsic source ellipticity are presented. We then apply our results to the FIRST radio-source survey. We predict an rms ellipticity of roughly 0.011 in 1xa0×xa01 deg2 pixels and 0.018 in 20xa0×xa020 arcmin2 pixels if the power spectrum is normalized to σ8xa0Ω0.53xa0=xa00.6, as indicated by the cluster abundance. The signal is significantly larger in some models if the power spectrum is normalized instead to the COBE anisotropy. The uncertainty in the predictions from imprecise knowledge of the FIRST redshift distribution is about 25 per cent in the rms ellipticity. We show that FIRST should be able to make a statistically significant detection of a weak-lensing signal for cluster-abundance-normalized power spectra.

Extragalactic Foregrounds of the Cosmic Microwave Background: Prospects for the MAP Mission

While the major contribution to the cosmic microwave background (CMB) anisotropies are the sought-after primordial fluctuations produced at the surface of last scattering, other effects produce secondary fluctuations at lower redshifts. Here we study the extragalactic foregrounds of the CMB in the context of the upcoming Microwave Anisotropy Probe (MAP) mission. We first survey the major extragalactic foregrounds and show that discrete sources, the Sunyaev-Zeldovich (SZ) effect, and gravitational lensing are the most dominant ones for MAP. We then show that MAP is expected to detect (>5 σ) about 46 discrete sources directly with 94 GHz fluxes above 2 Jy. The most prominent SZ features on the CMB sky are rich clusters of galaxies. In particular, we show that the Coma Cluster will be clearly detected and marginally resolved by MAP. We then consider a cosmological population of clusters, and show that MAP should detect (>5 σ) about 10 SZ clusters directly. The mean SZ fluxes of fainter clusters can be probed by cross-correlating MAP with cluster positions extracted from existing catalogs. For instance, a MAP-XBACs cross-correlation will be sensitive to clusters with S(94 GHz) 200 mJy, and will thus provide a test of their virialization state and a measurement of their gas fraction. Finally, we consider probing the hot gas on supercluster scales by cross-correlating the CMB with galaxy catalogs. Assuming that galaxies trace the gas, we show that a cross-correlation between the (MAP) and the Automatic Plate Measuring Facility (APM) catalog should yield a marginal detection, or at least a fourfold improvement on the COBE upper limits for the rms Compton y parameter.