arXiv subjects

Martin White

Publications and source records attributed to Martin White.

At least 289 records · Page 16Linked to original sources

arXiv2006

Simulations of Baryon Oscillations

The coupling of photons and baryons by Thomson scattering in the early universe imprints features in both the Cosmic Microwave Background (CMB) and matter power spectra. The former have been used to constrain a host of cosmological parameters, the latter have the potential to strongly constrain the expansion history of the universe and dark energy. Key to this program is the means to localize the primordial features in observations of galaxy spectra which necessarily involve galaxy bias, non-linear evolution and redshift space distortions. We present calculations, based on mock catalogs produced from high-resolution N-body simulations, which show the range of behaviors we might expect of galaxies in the real universe. We investigate physically motivated fitting forms which include the effects of non-linearity, galaxy bias and redshift space distortions and discuss methods for analysis of upcoming data. In agreement with earlier work, we find that a survey of several Gpc^3 would constrain the sound horizon at z~1 to about 1%.

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arXiv2006

Cosmological science enabled by Planck

Planck will be the first mission to map the entire cosmic microwave background (CMB) sky with mJy sensitivity and resolution better than 10'. The science enabled by such a mission spans many areas of astrophysics and cosmology. In particular it will lead to a revolution in our understanding of primary and secondary CMB anisotropies, the constraints on many key cosmological parameters will be improved by almost an order of magnitude (to sub-percent levels) and the shape and amplitude of the mass power spectrum at high redshift will be tightly constrained.

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arXiv2006

Redshift and Shear Calibration: Impact on Cosmic Shear Studies and Survey Design

The cosmological interpretation of weak lensing by large-scale structures requires knowledge of the redshift distribution of the source galaxies. Current lensing surveys are often calibrated using external redshift samples which span a significantly smaller sky area in comparison to the lensing survey, and are thus subject to sample variance. Some future lensing surveys are expected to be calibrated in the same way, in particular the fainter galaxy populations where the entire color coverage, and hence photometric redshift estimate, could be challenging to obtain. Using N-body simulations, we study the impact of this sample variance on cosmic shear analysis and show that, to first approximation, it behaves like a shear calibration error 1+/-epsilon. Using the Hubble Deep Field as a redshift calibration survey could therefore be a problem for current lensing surveys. We discuss the impact of the redshift distribution sampling error and a shear calibration error on the design of future lensing surveys, and find that a lensing survey of area Theta square degrees and limiting magnitude m_lim}, has a minimum shear and redshift calibration accuracy requirements given by epsilon = epsilon_0 10^{beta(m_lim-24.5)} / sqrt(Theta/ 200). Above that limit, lensing surveys would not reach their full potential. Using the galaxy number counts from the Hubble Ultra-Deep Field, we find (epsilon_0,beta)=(0.015,-0.18) and (epsilon_0,beta)=(0.011,-0.23) for ground and space based surveys respectively. Lensing surveys with no or limited redshift information and/or poor shear calibration accuracy will loose their potential to analyse the cosmic shear signal in the sub-degree angular scales, and therefore complete photometric redshift coverage should be a top priority for future lensing surveys.

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arXiv2006

Mosaicking with cosmic microwave background interferometers

Measurements of cosmic microwave background (CMB) anisotropies by interferometers offer several advantages over single-dish observations. The formalism for analyzing interferometer CMB data is well developed in the flat-sky approximation, valid for small fields of view. As the area of sky is increased to obtain finer spectral resolution, this approximation needs to be relaxed. We extend the formalism for CMB interferometry, including both temperature and polarization, to mosaics of observations covering arbitrarily large areas of the sky, with each individual pointing lying within the flat-sky approximation. We present a method for computing the correlation between visibilities with arbitrary pointing centers and baselines and illustrate the effects of sky curvature on the l-space resolution that can be obtained from a mosaic.

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arXiv2006

Potential sources of contamination to weak lensing measurements: constraints from N-body simulations

We investigate the expected correlation between the weak gravitational shear of distant galaxies and the orientation of foreground galaxies, through the use of numerical simulations. This shear-ellipticity correlation can mimic a cosmological weak lensing signal, and is potentially the limiting physical systematic effect for cosmology with future high-precision weak lensing surveys. We find that, if uncorrected, the shear-ellipticity correlation could contribute up to 10% of the weak lensing signal on scales up to 20 arcminutes, for lensing surveys with a median depth z=1. The most massive foreground galaxies are expected to cause the largest correlations, a result also seen in the Sloan Digital Sky Survey. We find that the redshift dependence of the effect is proportional to the lensing efficiency of the foreground, and this offers prospects for removal to high precision, although with some model dependence. The contamination is characterised by a weakly negative B-mode, which can be used as a diagnostic of systematic errors. We also provide more accurate predictions for a second potential source of error, the intrinsic alignment of nearby galaxies. This source of contamination is less important, however, as it can be easily removed with distance information.

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arXiv2006

On the Robustness of the Acoustic Scale in the Low-Redshift Clustering of Matter

We discuss the effects of non-linear structure formation on the signature of acoustic oscillations in the late-time galaxy distribution. We argue that the dominant non-linear effect is the differential motion of pairs of tracers separated by 150 Mpc. These motions are driven by bulk flows and cluster formation and are much smaller than the acoustic scale itself. We present a model for the non-linear evolution based on the distribution of pairwise Lagrangian displacements that provides a quantitative model for the degradation of the acoustic signature, even for biased tracers in redshift space. The Lagrangian displacement distribution can be calibrated with a significantly smaller set of simulations than would be needed to construct a precise power spectrum. By connecting the acoustic signature in the Fourier basis with that in the configuration basis, we show that the acoustic signature is more robust than the usual Fourier-space intuition would suggest because the beat frequency between the peaks and troughs of the acoustic oscillations is a very small wavenumber that is well inside the linear regime. We argue that any possible shift of the acoustic scale is related to infall on 150 Mpc scale, which is O(0.5%) fractionally at first-order even at z=0. For the matter, there is a first-order cancellation such that the mean shift is O(10^{-4}). However, galaxy bias can circumvent this cancellation and produce a sub-percent systematic bias.

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arXiv2005

Scale-dependent bias and the halo model

We use a simplified version of the halo model with a power law power spectrum to study scale dependence in galaxy bias at the very large scales relevant to baryon oscillations. In addition to providing a useful pedagogical explanation of the scale dependence of galaxy bias, the model provides an analytic tool for studying how changes in the Halo Occupation Distribution (HOD) impact the scale dependence of galaxy bias on scales between 10 and 1000 Mpc/h, which is useful for interpreting the results of complex N-body simulations. We find that changing the mean number of galaxies per halo of a given mass will change the scale dependence of the bias, but that changing the way the galaxies are distributed within the halo has a smaller effect on the scale dependence of bias at large scales. We use the model to explain the decay in amplitude of the baryon oscillations as k increases, and generalize the model to make predictions about scale dependent galaxy bias when redshift space distortions are introduced.

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arXiv2005

The DEEP2 Galaxy Redshift Survey: The Evolution of Void Statistics from z~1 to z~0

We present measurements of the void probability function (VPF) at z~1 using data from the DEEP2 Redshift Survey and its evolution to z~0 using data from the Sloan Digital Sky Survey (SDSS). We measure the VPF as a function of galaxy color and luminosity in both surveys and find that it mimics trends displayed in the two-point correlation function, $ξ$; namely that samples of brighter, red galaxies have larger voids (i.e. are more strongly clustered) than fainter, blue galaxies. We also clearly detect evolution in the VPF with cosmic time, with voids being larger in comoving units at z~0. We find that the reduced VPF matches the predictions of a `negative binomial' model for galaxies of all colors, luminosities, and redshifts studied. This model lacks a physical motivation, but produces a simple analytic prediction for sources of any number density and integrated two-point correlation function, \barξ. This implies that differences in the VPF across different galaxy populations are consistent with being due entirely to differences in the population number density and \barξ. The robust result that all galaxy populations follow the negative binomial model appears to be due to primarily to the clustering of dark matter halos. The reduced VPF is insensitive to changes in the parameters of the halo occupation distribution, in the sense that halo models with the same \barξ will produce the same VPF. For the wide range of galaxies studied, the VPF therefore does not appear to provide useful constraints on galaxy evolution models that cannot be gleaned from studies of \barξ alone. (abridged)

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arXiv2005

Characterizing the Shapes of Galaxy Clusters Using Moments of the Gravitational Lensing Shear

We explore the use of the tangential component of weak lensing shear to characterize the ellipticity of clusters of galaxies. We introduce an ellipticity estimator, and quantify its properties for isolated clusters from LCDM N-body simulations. We compare the N-body results to results from smooth analytic models. The expected distribution of the estimator for mock observations is presented, and we show how this distribution is impacted by contaminants such as noise, line of sight projections, and misalignment of the central galaxy used to determine the orientation of the triaxial halo. We examine the radial profile of the estimator and discuss tradeoffs in the observational strategy to determine cluster shape.

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arXiv2005

Going Nonlinear with Dark Energy Cosmologies

We propose an efficient method for generating high accuracy (~1%) nonlinear power spectra for grids of dark energy cosmologies. Our prescription for matching matter growth automatically matches the main features of the cosmic microwave background anisotropy power spectrum, thus naturally including ``CMB priors''.

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arXiv2005

Nulling Tomography with Weak Gravitational Lensing

We explore several strategies of eliminating (or nulling) the small-scale information in weak lensing convergence power spectrum measurements in order to protect against undesirable effects, for example the effects of baryonic cooling and pressure forces on the distribution of large-scale structures. We selectively throw out the small-scale information in the convergence power spectrum that is most sensitive to the unwanted bias, while trying to retain most of the sensitivity to cosmological parameters. The strategies are effective in the difficult but realistic situations when we are able to guess the form of the contaminating effect only approximately. However, we also find that the simplest scheme of simply not using information from the largest multipoles works about as well as the proposed techniques in most, although not all, realistic cases. We advocate further exploration of nulling techniques and believe that they will find important applications in the weak lensing data mining.

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arXiv2005

Reduced Shear Power Spectrum

Measurements of ellipticities of background galaxies are sensitive to the reduced shear, the cosmic shear divided by $(1-κ)$ where $κ$ is the projected density field. We compute the difference between shear and reduced shear both analytically and with simulations. The difference becomes more important on smaller scales, and will impact cosmological parameter estimation from upcoming experiments. A simple recipe is presented to carry out the required correction.

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arXiv2005

The Formation Histories of Galaxy Clusters

A sample of hundreds of simulated galaxy clusters is used to study the statistical properties of galaxy cluster formation. Individual assembly histories are discussed, the degree of virialization is demonstrated and various commonly used formation times are measured and inter-compared. In addition, the fraction of clusters which have ``recently'' undergone a major merger or significant mass jump is calculated as a function of lookback time and interval. The fraction of three- and four-body mergers is also studied.

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arXiv2005

Baryon oscillations

The coupling of photons and baryons by Thomson scattering in the early universe imprints features in both the Cosmic Microwave Background (CMB) and matter power spectra. The former have been used to constrain a host of cosmological parameters, the latter have the potential to strongly constrain the expansion history of the universe and dark energy. Key to this program is the means to localize the primordial features in observations of galaxy spectra which necessarily involve galaxy bias, non-linear evolution and redshift space distortions. We present calculations, based on mock catalogues produced from particle-mesh simulations, which show the range of behaviors we might expect of galaxies in the real universe. We find that non-linearity, galaxy bias and redshift space distortions all introduce important modifications to the basic picture. Both the galaxy bias and the (isotropic) redshift space distortions lead to relatively smooth modulations in power on the scales of interest to baryon oscillations. The halo and galaxy power spectra exhibit low order structure beyond the acoustic oscillations over the range of scales of relevance. Fitting a cubic polynomial to the ratio of galaxy to dark matter power reduces any remaining structure in the range 0.03<k<0.3 h/Mpc below the 2 per cent level, which is close to the error in our calculations.

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arXiv2005

Cluster arc statistics

We study the strong gravitational lensing properties of galaxy clusters obtained from N-body simulations with standard $Λ$CDM cosmology. We have used the 32 most massive clusters from a simulation at various redshifts and ray-traced through the clusters to investigate the giant arcs statistics. We have investigated the prevalence of multiple arc system, by looking at the multiple arc fraction (defined in the paper) systematically in various clusters and we have found that $\sim 40-50%$ of the clusters that produce giant arcs give multiple arcs, which agrees with the RCS{\sc ii} observations. We have also investigated the mass distributions that are efficient in lensing, discussed effects of source sizes and various other factors that are very important in the formation of giant arcs.

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arXiv2005

Reducing the shear

As gravitational lensing measurements become increasingly precise, it becomes necessary to include ever higher order effects in the theoretical calculations. Here we show how the difference between the shear and the reduced shear manifest themselves in a number of commonly used measures of shear power. If we are to reap the science rewards of future, high precision measurements of cosmic shear we will need to include this effect in our theoretical predictions.

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arXiv2005

Finding Clusters in SZ Surveys

We use simulated maps to investigate the ability of high resolution, low noise surveys of the CMB to create catalogues of Clusters of galaxies by detecting the characteristic signature imprinted by the Sunyaev Zeldovich effect. We compute the completeness of the catalogues in our simulations for several survey strategies, and evaluate the relative merit of some Fourier and wavelet based filtering techniques.

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arXiv2004

Using Weak Lensing to find Halo Masses

Since the strength of weak gravitational lensing is proportional to the mass along the line of sight, it might be possible to use lensing data to find the masses of individual dark matter clusters. Unfortunately, the effect on the lensing field of other matter along the line of sight is substantial. We investigate to what extent we can correct for these projection effects if we have additional information about the most massive halos along the line of sight from deep optical data. We conclude that unless we know the masses and positions of halos down to a very low mass, we can only correct for a small part of the line-of-sight projection, which makes it very hard to get accurate mass estimates of individual halos from lensing data.

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