arXiv subjects

Martin White

Publications and source records attributed to Martin White.

At least 343 records · Page 19Linked to original sources

arXiv2001

A Comparison of Simulated and Analytic Major Merger Counts

We use large volume, high resolution, N-body simulations of 3 different $Λ$CDM models, with different clustering strengths, to generate dark matter halo merging histories. Over the reliable range of halo masses, roughly galaxy groups to rich clusters of galaxies, we quantify the number density of major mergers for two different time intervals and compare with analytic predictions based on the extended Press-Schechter (1974) theory.

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arXiv2001

The Effect of the Cosmic Web on Cluster Weak Lensing Mass Estimates

In modern hierarchical theories of structure formation, rich clusters of galaxies form at the vertices of a weblike distribution of matter, with filaments emanating from them to large distances and with smaller objects forming and draining in along these filaments. The amount of mass contained in structure near the cluster can be comparable to the collapsed mass of the cluster itself. As the lensing kernel is quite broad along the line of sight around cluster lenses with typical redshifts near z=0.5, structures many Mpc away from the cluster are essentially at the same location as the cluster itself, when considering their effect on the cluster's weak lensing signal. We use large-scale numerical simulations of structure formation in a Lambda-dominated cold dark matter model to quantify the effect that large-scale structure near clusters has upon the cluster masses deduced from weak lensing analysis. A correction for the scatter in possible observed lensing masses should be included when interpreting mass functions from weak lensing surveys.

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arXiv2001

Constraints from the damping tail

The detection of anisotropy in the cosmic microwave background on arcminute scales by the Cosmic Background Imager (CBI) provides us with our first measurement of the damping tail and closes one chapter in the CMB story. We now have experimental verification for all of the features in the temperature anisotropy spectrum predicted theoretically two decades ago. The CBI result allows us to constrain both parameterized models based on the inflationary cold dark matter (CDM) paradigm and to examine model independent constraints on the matter content, the distance to last scattering and the thickness of the last scattering surface. By providing a lower limit on the duration of recombination it implies a lower limit on the polarization of the sub-degree scale anisotropy which is close to current experimental upper limits.

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arXiv2001

Global Probes of the Impact of Baryons on Dark Matter Halos

The halo mass function, dn/dM, predicted by hierarchical clustering models can be measured indirectly using dynamical probes like the distribution of gravitational lens image separations, dn/dDelta-theta, or halo circular velocities, dn/d v_c. These dynamical variables depend on the halo structure as well as the halo mass. Since baryonic physics, particularly cooling, significantly modifies the central density structure of dark matter halos, both observational distributions show a feature corresponding to the mass scale below which the baryons in the halo can cool (i.e. galaxies versus clusters). We use simplified but self-consistent models to show that the structural changes to the halos produced by the cooling baryons explain both distributions. Given a fixed halo mass function, matching the observed image separation distribution or local velocity function depends largely on Omega_b through its effects on the cooling time scales. These baryonic effects on the halo structure also affect the evolution of the velocity function of galaxies with redshift.

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arXiv2000

The mass of a halo

We discuss the different definitions of the mass of a halo in common use and how one may convert between them. Using N-body simulations we show that mass estimates based on spherical averages are much more tightly correlated with each other than with masses based on the number of particles in a halo. The mass functions pertaining to some different mass definitions are estimated and compared to the `universal form' of Jenkins et al. (2000). Using a different simulation pipeline and a different cosmological model we show that the mass function is well fit by the Jenkins et al. (2000) fitting function, strengthening the claim to universality made by those authors. We show that care must be taken to match the definitions of mass when using large N-body simulations to bootstrap scaling relations from smaller hydrodynamical runs to avoid observationally significant bias in the predictions for abundances of objects.

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arXiv2000

Power Spectra Estimation for Weak Lensing

We develop a method for estimating the shear power spectra from weak lensing observations and test it on simulated data. Our method describes the shear field in terms of angular power spectra and cross correlation of the two shear modes which differ under parity transformations. Two of the three power spectra can be used to monitor unknown sources of noise in the data. The power spectra are decomposed in a model independent manner in terms of ``band-powers'' which are then extracted from the data using a quadratic estimator to find the maximum of the likelihood and its local curvature (for error estimates). We test the method against simulated data from Gaussian realizations and cosmological N-body simulations. In the Gaussian case, the mean bandpowers and their covariance are well recovered even for irregular (or sparsely) sampled fields. The mild non-Gaussianity of the N-body realizations causes a slight underestimation of the errors that becomes negligible for scales much larger than several arcminutes and does not bias the recovered band powers.

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arXiv2000

Boomerang returns unexpectedly

Experimental study of the anisotropy in the cosmic microwave background (CMB) is gathering momentum. The eagerly awaited Boomerang results have lived up to expectations. They provide convincing evidence in favor of the standard paradigm: the Universe is close to flat and with primordial fluctuations which are redolent of inflation. Further scrutiny reveals something even more exciting however -- two hints that there may be some unforeseen physical effects. Firstly the primary acoustic peak appears at slightly larger scales than expected. Although this may be explicable through a combination of mundane effects, we suggest it is also prudent to consider the possibility that the Universe might be marginally closed. The other hint is provided by a second peak which appears less prominent than expected. This may indicate one of a number of possibilities, including increased damping length or tilted initial conditions, but also breaking of coherence or features in the initial power spectrum. Further data should test whether the current concordance model needs only to be tweaked, or to be enhanced in some fundamental way.

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arXiv2000

The redshift space power spectrum in the halo model

Recently there has been a lot of attention focussed on a virialized halo-based approach to understanding the properties of the matter and galaxy power spectrum. We show that this model allows a natural treatment of the large and small scale redshift space distortions, which we develop here, which extends the pedagogical value of the approach.

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arXiv2000

Still flat after all these years

The Universe could be spatially flat, positively curved or negatively curved. Each option has been popular at various times, partly affected by an understanding that models tend to evolve away from flatness. The curvature of the Universe is amenable to measurement, through tests such as the determination of the angles of sufficiently large triangles. The angle subtended by the characteristic scale on the Cosmic Microwave sky provides a direct test, which has now been realised through a combination of exquisite results from a number of CMB experiments. After a long and detailed investigation, with many false clues, it seems that the mystery of the curvature of the Universe is now solved. It's an open and shut case: the Universe is flat.

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arXiv2000

A Quantitative Study of Interacting Dark Matter in Halos

We study the evolution of Hernquist profile ``galaxies'' in the presence of self-interacting dark matter (SIDM), where the properties of the dark matter can be parameterized by one number, sigma-hat = sigma M/a^2 for a halo of mass M and break radius a. While the halos form constant density cores of size a/2 on the core radius relaxation time scale core collapse begins shortly thereafter and a steeper 1/r^2 central density cusp starts forming faster than predicted by 2-body relaxation. The formation of the steeper central cusp is accelerated if the cooling baryons adiabatically compress the dark matter. The natural consequence of SIDM is to exacerbate rather than to mitigate astrophysical problems created by dark matter density cusps.

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arXiv2000

A Characteristic Scale on the Cosmic Microwave Sky

The current suite of results from Cosmic Microwave Background anisotropy experiments is fulfilling the promise of providing extraordinary levels of discrimination between cosmological models. We calculate a binned anisotropy power spectrum, which we tabulate, along with error bars and bin-to-bin correlations, so that it can be easily used for constraining models. The resulting power spectrum is flat at large angles, with a gradual rise to a prominent peak at around 0.5 degrees and a decrease thereafter. This is precisely the shape predicted by inflationary-inspired adiabatic models. Within that class of cosmologies, this characteristic scale imprinted on the CMB sky can be used to infer that the geometry of the Universe is very close to flat. The next wave of CMB results should add fuel to the debate about whether or not the Universe once inflated, as well as beginning in earnest the task of measuring cosmological parameters.

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arXiv2000

Suppressing Linear Power on Dwarf Galaxy Halo Scales

Recently is has been suggested that the dearth of small halos around the Milky Way arises due to a modification of the primordial power spectrum of fluctuations from inflation. Such modifications would be expected to alter the formation of structure from bottom-up to top-down on scales near where the short-scale power has been suppressed. Using cosmological simulations we study the effects of such a modification of the initial power spectrum. While the halo multiplicity function depends primarily on the linear theory power spectrum, most other probes of power are more sensitive to the non-linear power spectrum. Collapse of large-scale structures as they go non-linear regenerates a ``tail'' in the power spectrum, masking small-scale modifications to the primordial power spectrum except at very high-z. Even the small-scale (k>2h/Mpc) clustering of the Ly-alpha forest is affected by this process, so that CDM models with sufficient power suppression to reduce the number of 10^10 Msun halos by a factor of about 5 give similar Ly-alpha forest power spectrum results. We conclude that other observations that depend more directly on the number density of collapsed objects, such as the number of damped Ly-alpha systems, or the redshift of reionization may provide the most sensitive tests of these models.

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arXiv2000

A New Algorithm for Computing Statistics of Weak Lensing by Large-Scale Structure

We describe an efficient algorithm for calculating the statistics of weak lensing by large-scale structure based on a tiled set of independent particle-mesh N-body simulations which telescope in resolution along the line of sight. This efficiency allows us to predict not only the mean properties of lensing observables such as the power spectrum, skewness and kurtosis of the convergence, but also their sampling errors for finite fields of view, which are themselves crucial for assessing the cosmological significance of observations. We find that the nongaussianity of the distribution substantially increases the sampling errors for the skewness and kurtosis in the several to tens of arcminutes regime, whereas those for the power spectrum are only fractionally increased even out to wavenumbers where shot noise from the intrinsic ellipticities of the galaxies will likely dominate the errors.

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arXiv1999

Analyzing data from DASI

We present an optimized layout of horn positions for the Degree Angular Scale Interferometer (DASI), which provides good coverage of the u-v plane and 3-fold symmetry. We investigate how an optimal subspace filtering analysis of a single pointing of DASI could be used to determine the anisotropies in the cosmic microwave background over a range of angular scales near 30'. We discuss the complementarity between the angular scales probed by DASI and NASA's MAP satellite at 30GHz and strategies for imaging the sky at these frequencies. Finally, we quantitatively assess how well the angular power spectrum can be recovered by deconvolution of linear combinations of squares of the DASI visibilities.

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arXiv1999

Limits on the gravity wave contribution to microwave anisotropies

We present limits on the fraction of large angle microwave anisotropies which could come from tensor perturbations. We use the COBE results as well as smaller scale CMB observations, measurements of galaxy correlations, abundances of galaxy clusters, and Lyman alpha absorption cloud statistics. Our aim is to provide conservative limits on the tensor-to-scalar ratio for standard inflationary models. For power-law inflation, for example, we find T/S<0.52 at 95% confidence, with a similar constraint for phi^p potentials. However, for models with tensor amplitude unrelated to the scalar spectral index it is still currently possible to have T/S>1.

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arXiv1999

Higher order moments of the density field in a parameterized sequence of non-gaussian theories

We calculate the higher order moments in a sequence of models where the initial density fluctuations are drawn from a chi^2_nu distribution with a power-law power spectrum. For large values of nu the distribution is approximately gaussian, and we reproduce the values known from perturbation theory. As νis lowered the distribution becomes progressively more non-gaussian, approximating models with rare, high-amplitude peaks. The limit nu=1 is a realization of recently proposed isocurvature models for producing early galaxy formation where the density perturbations are quadratic in a gaussian field.

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arXiv1999

The Growth of Correlations in the Matter Power Spectrum

We find statistically significant correlations in the cosmological matter power spectrum over the full range of observable scales. While the correlations between individual modes are weak, the band-averaged power spectrum shows strong non-trivial correlations. The correlations are significant when the modes in either one or both bands are in the non-linear regime, and approach 100% for pairs of bands in which all the modes are non-linear. The correlations are weaker, but not absent, when computed in redshift space. Since estimates of the power spectrum from galaxy surveys require band-averaging, the correlations must be taken into account when comparing a measured power spectrum with theoretical models.

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arXiv1999

Gravity waves goodbye

The detection of a stochastic background of long-wavelength gravitational waves (tensors) in the cosmic microwave background (CMB) anisotropy would be an invaluable probe of the high energy physics of the early universe. Unfortunately a combination of factors now makes such a detection seem unlikely: the vast majority of the CMB signal appears to come from density perturbations (scalars) - detailed fits to current observations indicate a tensor-to-scalar quadrupole ratio of T/S < 0.5 for the simplest models; and on the theoretical side the best-motivated inflationary models seem to require very small T/S. Unfortunately CMB temperature anisotropies can only probe a gravity wave signal down to T/S \sim 10% and optimistic assumptions about polarization of the CMB only lower this another order of magnitude.

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