arXiv subjects

Martin White

Publications and source records attributed to Martin White.

At least 307 records · Page 17Linked to original sources

arXiv2004

Tomography of lensing cross power spectra

By obtaining photometric redshift information, tomography allows us to cross-correlate galaxy ellipticities in different source redshift bins. The cross-correlation is non-vanishing because the different bins share much of the foreground mass distribution from which, over Gpc scales, the lensing signal is built. If the redshift bins are thick enough however, the cross-correlations are insensitive to contamination from the intrinsic alignments of galaxies since these fall off rapidly on scales larger than a few tens of Mpc. We forecast how lensing tomography using only the cross-power spectra can constrain cosmological parameters compared to tomography including the auto-spectra. It is shown that the parameter errors are degraded by only O(10%) for 5 or more source redshift bins. Thus, the cross-power spectrum tomography can be a simple, model-independent means of reducing the intrinsic alignment contamination while retaining most of the constraints on cosmology.

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arXiv2004

Theoretical uncertainty in baryon oscillations

We discuss the systematic uncertainties in the recovery of dark energy properties from the use of baryon acoustic oscillations as a standard ruler. We demonstrate that while unknown relativistic components in the universe prior to recombination would alter the sound speed, the inferences for dark energy from low-redshift surveys are unchanged so long as the microwave background anisotropies can measure the redshift of matter-radiation equality, which they can do to sufficient accuracy. The mismeasurement of the radiation and matter densities themselves (as opposed to their ratio) would manifest as an incorrect prediction for the Hubble constant at low redshift. In addition, these anomalies do produce subtle but detectable features in the microwave anisotropies.

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arXiv2004

Baryons and weak lensing power spectra

We provide a simple toy model to elucidate the effects of baryonic cooling on predictions of the convergence power spectrum in weak gravitational lensing. Our model suggests that the effects of baryonic cooling on dark matter halo profiles can alter the dark matter power spectrum at k~10 h/Mpc and lead to percent level changes in the convergence angular power spectrum at wavenumbers of several thousand. These effects would be measurable with planned weak lensing surveys and will impact constraints on cosmological parameters.

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arXiv2004

Weak Lensing of the CMB by Large-Scale Structure

Several recent papers have studied lensing of the CMB by large-scale structures, which probes the projected matter distribution from $z=10^3$ to $z\simeq 0$. This interest is motivated in part by upcoming high resolution, high sensitivity CMB experiments, such as APEX/SZ, ACT, SPT or Planck, which should be sensitive to lensing. In this paper we examine the reconstruction of the large-scale dark matter distribution from lensed CMB temperature anisotropies. We go beyond previous work in using numerical simulations to include higher order, non-Gaussian effects and study how well the quadratic estimator of \cite{Hu01a} is able to recover the input field. We also study contamination by kinetic Sunyaev-Zel'dovich signals, which is spectrally indistinguishable from lensed CMB anisotropies. We finish by estimating the sensitivity of the previously cited experiments.

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arXiv2004

Simulations of weak gravitational lensing

We describe the simulation data produced by a pilot programme to compute mock weak gravitational lensing maps for a range of currently popular cosmological models by ray tracing through high-resolution N-body simulations. The programme required only a modest investment in computer time to produce maps accurate to arcminute scales covering hundreds of square degrees of sky for 4 cosmological models.

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arXiv2004

Large scale systematic signals in weak lensing surveys

We use numerical simulations to model the effect of seeing and extinction modulations on weak lensing surveys. We find that systematic fluctuations in the shear amplitude and source depth can give rise to changes in the $E$-mode signal and to varying amplitudes of large scale $B$-modes. Exquisite control of such systematics will be required as we approach the era of precision cosmology with weak lensing.

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arXiv2004

Mock Catalogs for the DEEP2 redshift survey

We present a set of mock redshift catalogs, constructed from N-body simulations, designed to mimic the DEEP2 survey. Galaxies with a range of luminosities are placed within virialized halos in the simulation using a variant of the halo model. The halo model parameters are chosen to reproduce local clustering and abundance data and assumed to be independent of redshift. This allows us to predict the luminosity function, two-point correlation function, luminosity- and scale-dependent bias, redshift space distortions etc. of our galaxies at higher redshifts. We show that the low order clustering properties are consistent with preliminary DEEP2 data. The catalogs can be used to evaluate the selection effects of the survey and to test new algorithms and statistics that are to be used in the analysis of DEEP2 data.

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arXiv2004

Halo asphericity and the shear 3-pt function

We demonstrate through the use of a simple toy model that asphericity in dark matter halos has a measurable effect on the configuration dependence of the weak lensing shear 3-point function at small scales. This sensitivity provides a way, in principle, to measure the shapes of dark matter halos. The distribution of halo ellipticities should be included in models aiming at a high fidelity prediction of n-point shear correlation statistics.

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arXiv2004

Cluster Lensing of the CMB

We investigate what the lensing information contained in high resolution, low noise CMB temperature maps can teach us about cluster mass profiles. We create lensing fields and Sunyaev-Zel'dovich effect maps from N-body simulations and apply them to primary CMB anisotropies modeled as a Gaussian random field. We examine the success of several techniques of cluster mass reconstruction using CMB lensing information, and make an estimate of the observational requirements necessary to achieve a satisfactory result.

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arXiv2003

Radiative transfer through the Intergalactic Medium

We use a probabilistic method to compute the propagation of an ionization front corresponding to the re-ionization of the intergalactic medium in a LCDM cosmology, including both hydrogen and helium. The effects of radiative transfer substantially boost the temperature of the ionized gas over the case of uniform re-ionization. The resulting temperature-density relation of the ionized gas is both non-monotonic and multiple-valued, reflecting the non-local character of radiative transfer and suggesting that a single polytropic relation between local gas density and temperatue is a poor description of the thermodynamic state of baryons in the post-reionization universe.

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arXiv2003

Point sources in the context of future SZ surveys

We look at the impact of Infra-Red (IR) and radio point sources on upcoming large yield Sunyaev-Zel'dovich (SZ) cluster surveys such as APEX, SPT and ACT. The IR and radio point source counts are based on observations by the the SCUBA and WMAP instruments respectively. We show that the contributions from IR source counts, when extrapolated from the SCUBA frequency of 350GHz to the operating frequencies of these surveys (~100-300GHz), can be a significant source of additional `noise' which needs to be accounted for in order to extract the optimal science from these surveys. Alternatively, these surveys give us an opportunity to study IR sources, their numbers and clustering properties, opening a new window to the high redshift universe. For the radio point sources, the contribution depends on a more uncertain extrapolation from 40GHz but is comparable to the IR near 200GHz. However, the radio signal may be correlated with clusters of galaxies and have a disproportionately larger effect.

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arXiv2003

Constraining evolution in the Halo Model using galaxy redshift surveys

We use the latest observations from the 2dF Galaxy Redshift Survey to fit the conditional luminosity function (CLF) formulation of the halo-model for galaxies at z=0. This fit is then used to test the extent of evolution in the halo occupation distribution (HOD) to z=0.8, by comparing the predicted clustering from this CLF to preliminary results from the DEEP2 Redshift Survey. We show that the current observations from the DEEP2 Redshift Survey are remarkably consistent with no evolution in the CLF from z=0 to z=0.8. This result is surprising, in that it suggests that there has been very little change in the way galaxies occupy their host dark matter halos over half the age of the Universe. We discuss in detail the observational constraints we have adopted and also the various different selection effects in each survey and how these impact on the galaxy populations encountered in each survey.

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arXiv2003

Studying clusters with Planck

We use mock Sunyaev-Zel'dovich effect (SZE) maps to investigate how well the Planck mission might find and characterize clusters of galaxies. We discuss different combinations of frequency maps and different methods for identifying cluster candidates. The resulting catalogues are not complete even for relatively high mass thresholds, but the full sky nature of the mission ensures a large sample of massive, high-z clusters which will be ideal for many studies. We make a preliminary attempt to identify the X-ray, optical and weak lensing properties of the Planck sample.

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arXiv2003

The effects of UV background correlations on LyA forest flux statistics

We examine the possible effects of UV background fluctuations on the pixel flux power spectrum and auto-correlation function of the LyA forest due to a finite number of sources in an attenuating medium. We consider scenarios in which QSO sources dominate the contribution to the UV background. To estimate their contribution, we use the QSO luminosity functions from the 2dF and Sloan QSO surveys. We estimate self-consistent values for the attenuation length at the Lyman edge through the Intergalactic Medium using Particle Mesh simulations of the LyA forest, normalised by the measured mean LyA flux. We examine the convergence properties of the flux power spectrum and auto-correlation function under differing assumptions regarding pressure-smoothing. Convergence to better than 10% in the flux power spectrum at proper wavenumbers k < 0.015s/km requires a comoving resolution of 30-60kpc/h in a comoving box of size 25Mpc/h at z = 5, with the requirements becoming more severe at lower redshifts. We find much better covergence properties for the relative effects of the UV background fluctuations on the flux power spectrum. The background fluctuations increase the large-scale power and suppress the power at intermediate scales. At z < 4, the effect is only at the few percent level. By z > 5, however, the large-scale power is boosted by more than 50%, suggesting the flux power spectrum may serve as a useful tool for distinguishing QSO-dominated UV background scenarios from those dominated by galaxies.

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arXiv2003

Cosmology with the Ly-a forest

The Lya forest has emerged as one of the few systems capable of probing small-scale structure at high-z with high precision. In this talk I highlight two areas in which the Lya forest is shedding light on fundamental questions in cosmology, one speculative and one which should be possible in the near future.

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arXiv2003

Simulating weak lensing by large scale structure

We model weak gravitational lensing of light by large-scale structure using ray tracing through N-body simulations. The method is described with particular attention paid to numerical convergence. We investigate some of the key approximations in the multi-plane ray tracing algorithm. Our simulated shear and convergence maps are used to explore how well standard assumptions about weak lensing hold, especially near large peaks in the lensing signal.

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arXiv2003

Constraints on the UV metagalactic emissivity using the LyA forest

Numerical hydrodynamical simulations have proven a successful means of reproducing many of the statistical properties of the LyA forest as measured in high redshift (z > 4) quasar spectra. The source of ionization of the Intergalactic Medium (IGM), however, remains unknown. We investigate how the LyA forest may be used to probe the nature of the sources. We show that the attenuation of Lyman continuum photons by the IGM depends sensitively on the emissivity of the sources, permitting a strong constraint to be set on the required emissivity to match the measured values of the mean IGM LyA optical depth. We also investigate the effect of fluctuations in the UV background. We derive the distribution function of the background radiation field produced by discrete sources in an infinite universe, including the effects of attenuation by an intervening absorbing medium. We show that for z > 5, the fluctuations boost the mean LyA optical depth in a scenario in which QSO sources dominate the UV background. The fluctuations will also result in large spatial correlations in the ionization level of the IGM at high redshifts, providing a means of discriminating between scenarios in which stars and galaxies dominate the UV background vs a QSO-dominated background.

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arXiv2002

Prospects for detecting neutral hydrogen using 21cm radiation from large scale structure at high redshifts

We estimate the signal from large scale structure at high redshifts in redshifted 21cm line. We focus on z ~ 3 and the LCDM cosmology. We assume that neutral hydrogen is to be found only in galaxies, and normalise the total content to the density parameter of neutral hydrogen in damped Lyman alpha absorption systems (DLAS). We find that the rms fluctuations due to the large scale distribution of galaxies is very small and cannot be observed at angular scales probed by present day telescopes. We have used the sensitivity of the Giant meter-wave Radio Telescope (GMRT) for comparison. We find that observations stretching over 1000hours will be required for a 3 sigma detection of such objects.

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