Asantha R. Cooray

Power Spectrum Covariance of Weak Gravitational Lensing

Weak gravitational lensing observations probe the spectrum and evolution of density fluctuations and the cosmological parameters that govern them. At low redshifts, the nonlinear gravitational evolution of large-scale structure produces a non-Gaussian covariance in the shear power spectrum measurements that affects their translation into cosmological parameters. Using the dark matter halo approach, we study the covariance of binned band power spectrum estimates and the four-point function of the dark matter density field that underlies it. We compare this semianalytic estimate to results from N-body numerical simulations and find good agreement. We find that for a survey out to z ~ 1, the power spectrum covariance increases the errors on cosmological parameters determined under the Gaussian assumption by about 15%.

Gravitational Lensing as a Probe of Quintessence

A large number of cosmological studies now suggest that roughly two-thirds of the critical energy density of the universe exists in a component with negative pressure. If the equation of state of such an energy component varies with time, it should in principle be possible to identify such a variation using cosmological probes over a wide range in redshift. Proper detection of any time variation, however, requires cosmological probes beyond the currently studied range in redshift of ~0.1-1. We extend our analysis to gravitational lensing statistics at high redshift and suggest that a reliable sample of lensed sources, out to a redshift of ~5, can be used to constrain the variation of the equation of state, provided that both the redshift distribution of lensed sources and the selection function involved with the lensed source discovery process are known. An exciting opportunity to catalog an adequate sample of lensed sources (quasars) for probing quintessence is now available with the ongoing Sloan Digital Sky Survey. Writing w(z)≈w+z(dw/dz), we study the expected accuracy to which the equation of state today (w) and its rate of change [(dw/dz)] can simultaneously be constrained. Such a determination can rule out some missing-energy candidates, such as classes of quintessence models or a cosmological constant.

Imprint of Reionization on the Cosmic Microwave Background Bispectrum

We study contributions to the cosmic microwave background (CMB) bispectrum from non- Gaussianity induced by secondary anisotropies during reionization. Large-scale structure in the reionized epoch both gravitationally lenses CMB photons and produces Doppler shifts in their temperature from scattering oU electrons in infall. The resulting correlation is potentially observable through the CMB bispectrum. The second-order Ostriker-Vishniac eUect also couples to a variety of linear secondary eUects to produce a bispectrum. For the currently favored —at cosmological model with a low matter content and small optical depth in the reionized epoch however, these bispectrum contributions q ( 0.3, are well below the detection threshold of MAP and at or below that of Planck, given their cosmic and noise variance limitations. At the upper end of this range, they can serve as an extra source of noise for measurements with Planck of either primordial nongaussianity or that induced by the correlation of gravitational lensing with the integrated Sachs-Wolfe and the thermal Sunyaev-Zeldovich eUects. We include a discussion of the general properties of the CMB bispectrum, its con—guration dependence for the various eUects, and its computation in the Limber approximation and beyond. Subject headings: cosmic microwave backgroundcosmology: theory ¨ large-scale structure of universegravitational lensing

Large-Scale Sunyaev-Zeldovich Effect: Measuring Statistical Properties with Multifrequency Maps

We study the prospects for extracting detailed statistical properties of the Sunyaev-Zeldovich (SZ) eUect associated with large-scale structure using upcoming multifrequency cosmic microwave background (CMB) experiments. The greatest obstacle to detecting the large-angle signal is the confusion noise pro- vided by the primary anisotropies themselves, and to a lesser degree Galactic and extragalactic fore- grounds. We employ multifrequency subtraction techniques and the latest foregrounds models to determine the detection threshold for the Boomerang, Microwave Anisotropy Probe (MAP; several kK), and Planck CMB (sub-kK) experiments. Calibrating a simpli—ed biased-tracer model of the gas pressure from recent hydrodynamic simulations, we estimate the SZ power spectrum, skewness, and bispectrum through analytic scalings and N-body simulations of the dark matter. We show that the Planck satellite should be able to measure the SZ eUect with sufficient precision to determine its power spectrum and higher order correlations, e.g., the skewness and bispectrum. Planck should also be able to detect the cross-correlation between the SZ and gravitational lensing eUect in the CMB. Detection of these eUects will help determine the properties of the as yet undetected gas, including the manner in which the gas pressure traces the dark matter. Subject headings: cosmic microwave backgroundcosmology: theory ¨ large-scale structure of universe

The Sunyaev-Zeldovich Effect in Abell 370

We present interferometric measurements of the Sunyaev-Zeldovich (SZ) eUect toward the galaxy cluster Abell 370. These measurements, which directly probe the pressure of the clusters gas, show the gas distribution to be strongly aspherical, as do the X-ray and gravitational lensing observations. We calculate the clusters gas mass fraction in two ways. We —rst compare the gas mass derived from the SZ measurements to the lensing-derived gravitational mass near the critical lensing radius. We also calculate the gas mass fraction from the SZ data by deprojecting the three-dimensional gas density distribution and deriving the total mass under the assumption that the gas is in hydrostatic equilibrium (HSE). We test the assumptions in the HSE method by comparing the total cluster mass implied by the two methods and —nd that they agree within the errors of the measurement. We discuss the possible system- atic errors in the gas mass fraction measurement and the constraints it places on the matter density parameter, ) M. Subject headings: cosmic microwave backgroundcosmology: observations ¨ galaxies: clusters: individual (Abell 370) ¨ techniques: interferometric

WEAK LENSING BY LARGE-SCALE STRUCTURE: A DARK MATTER HALO APPROACH

Weak gravitational lensing observations probe the spectrum and evolution of density fluctuations and the cosmological parameters that govern them, but they are currently limited to small fields and subject to selection biases. We show how the expected signal from large-scale structure arises from the contributions from and correlations between individual halos. We determine the convergence power spectrum as a function of the maximum halo mass and so provide the means to interpret results from surveys that lack high-mass halos either through selection criteria or small fields. Since shot noise from rare massive halos is mainly responsible for the sample variance below 10&arcmin;, our method should aid our ability to extract cosmological information from small fields.

Weak Gravitational Lensing Bispectrum

Weak gravitational lensing observations probe the spectrum and evolution of density fluctuations and the cosmological parameters that govern them. The nonlinear evolution of large-scale structure produces a non-Gaussian signal that is potentially observable in galaxy shear data. We study the three-point statistics of the convergence, specifically the bispectrum, using the dark matter halo approach, which describes the density field in terms of correlations between and within dark matter halos. Our approach allows us to study the effect of the mass distribution in observed fields, in particular the bias induced by the lack of rare massive halos (clusters) in observed fields. We show that the convergence skewness is primarily due to rare and massive dark matter halos, with skewness converging to its mean value only if halos of mass M > 1015 M☉ are present. This calculational method can in principle be used to correct for such a bias as well as to search for more robust statistics related to the two- and three-point correlations.

Measuring Angular Diameter Distances through Halo Clustering

Current and upcoming wide-field surveys for weak gravitational lensing and the Sunyaev-Zeldovich effect will generate mass-selected catalogs of dark matter halos with internal or follow-up photometric redshift information. Using the shape of the linear power spectrum as a standard ruler that is calibrated by cosmic microwave background measurements, we find that a survey of 4000 deg^2 and a mass threshold of 10^(14) M_☉ can be used to determine the comoving angular diameter distance as a function of redshift. In principle, this test also allows an absolute calibration of the distance scale and measurement of the Hubble constant. This test is largely insensitive to the details of halo mass measurements, mass function, and halo bias. Determination of these quantities would further allow a measurement of the linear growth rate of fluctuations.

Non-Gaussian aspects of thermal and kinetic Sunyaev-Zel’dovich effects

We discuss non-Gaussian effects associated with the local large-scale structure contributions to the cosmic microwave background (CMB) anisotropies through the thermal Sunyaev-Zel’dovich (SZ) effect. The non-Gaussianities associated with the SZ effect arise from the existence of a significant four-point correlation function in large scale pressure fluctuations. Using the pressure trispectrum calculated under the recently popular halo model, we discuss the full covariance of the SZ thermal power spectrum. We use this full covariance matrix to study the astrophysical uses of the SZ effect and discuss the extent to which gas properties can be derived from a measurement of the SZ power spectrum. With the SZ thermal effect separated in temperature fluctuations using its frequency information, the kinetic SZ effect, also known as the Ostriker-Vishniac effect, is expected to dominate the CMB temperature fluctuations at small angular scales. This effect arises from the baryon modulation of the first order Doppler effect resulting from the relative motion of scatterers. The presence of the SZ kinetic effect can be determined through a cross-correlation between the SZ thermal and a CMB map at small scales. Since the SZ kinetic effect is second order, however, contributions to such a cross-correlation arise to lower order in the form of a three-point correlation function, or a bispectrum in Fourier space. We suggest an additional statistic that can be used to study the correlation between pressure traced by the SZ thermal effect and the baryons traced by the SZ kinetic effect involving the cross-power spectrum constructed through squared temperatures instead of the usual temperature itself. Through a signal-to-noise calculation, we show that future small angular scale multifrequency CMB experiments, sensitive to multipoles of a few thousand, will be able to measure the cross-correlation of SZ thermal and SZ kinetic effects through a temperature squared power spectrum.

Optical and near infrared observations of the afterglow of GRB 980329 from 15 hours to 10 days

We report I-band observations of the GRB 980329 field made on 1998 March 29 with the 1.34 m Tautenberg Schmidt telescope, R-, J- and K-band observations made on 1998 April 1 with the APO 3.5 m telescope, R- and I-band observations made on 1998 April 3 with the Mayall 4 m telescope at KPNO, and J- and K-band observations made 1998 April 6-8 with the Keck-I 10 m telescope. We show that these and other reported measurements are consistent with a power-law fading of the optical/near-infrared source that is coincident with the variable radio source VLA J0702+3850. This firmly establishes that this source is the afterglow of GRB 980329.