An Improved Semi-Analytical Spherical Collapse Model for Non-linear Density Evolution

We derive a semi-analytical extension of the spherical collapse model of structure formation that takes account of the effects of deviations from spherical symmetry and shell crossing which are important in the non-linear regime. Our model is designed so that it predicts a relation between the peculiar velocity and density contrast that agrees with the results of N-body simulations in the region where such a comparison can sensibly be made. Prior to turnaround, when the unmodified spherical collapse model is expect to be a good approximation, the predictions of the two models coincide almost exactly. The effects of a late time dominating dark energy component are also taken into account. The improved spherical collapse model is a useful tool when one requires a good approximation not just to the evolution of the density contrast but also its trajectory. Moreover, the analytical fitting formulae presented is simple enough to be used anywhere where the standard spherical collapse might be used but with the advantage that it includes a realistic model of the effects of virialisation.Comment: 6 pages, 3 figures. Matches the version in print at Astrophys.

Cosmic Mach Number as a Function of Overdensity and Galaxy Age

We carry out an extensive study of the cosmic Mach number (\mach) on scales of R=5, 10 and 20h^-1Mpc using an LCDM hydrodynamical simulation. We particularly put emphasis on the environmental dependence of \mach on overdensity, galaxy mass, and galaxy age. We start by discussing the difference in the resulting \mach according to different definitions of \mach and different methods of calculation. The simulated Mach numbers are slightly lower than the linear theory predictions even when a non-linear power spectrum was used in the calculation, reflecting the non-linear evolution in the simulation. We find that the observed \mach is higher than the simulated mean by more than 2-standard deviations, which suggests either that the Local Group is in a relatively low-density region or that the true value of \Omega_m is ~ 0.2, significantly lower than the simulated value of 0.37. We show from our simulation that the Mach number is a weakly decreasing function of overdensity. We also investigate the correlations between galaxy age, overdensity and \mach for two different samples of galaxies --- DWARFs and GIANTs. Older systems cluster in higher density regions with lower \mach, while younger ones tend to reside in lower density regions with larger \mach, as expected from the hierarchical structure formation scenario. However, for DWARFs, the correlation is weakened by the fact that some of the oldest DWARFs are left over in low-density regions during the structure formation history. For giant systems, one expects blue-selected samples to have higher \mach than red-selected ones. We briefly comment on the effect of the warm dark matter on the expected Mach number.Comment: 43 pages, including 15 figures. Accepted version in ApJ. Included correlation function of different samples of galaxies, and the cumulative number fraction distribution as a fcn. of overdensity. Reorganized figures and added some reference

Cosmological redshift distortion: deceleration, bias and density parameters from future redshift surveys of galaxies

The observed two-point correlation functions of galaxies in redshift space become anisotropic due to the geometry of the universe as well as due to the presence of the peculiar velocity field. On the basis of linear perturbation theory, we expand the induced anisotropies of the correlation functions with respect to the redshift $z$, and obtain analytic formulae to infer the deceleration parameter $q_0$, the density parameter $\Omega_0$ and the derivative of the bias parameter $d\ln b/dz$ at $z=0$ in terms of the observable statistical quantities. The present method does not require any assumption of the shape and amplitude of the underlying fluctuation spectrum, and thus can be applied to future redshift surveys of galaxies including the Sloan Digital Sky Survey. We also evaluate quantitatively the systematic error in estimating the value of $\beta_0 \equiv \Omega_0^{0.6}/b$ from a galaxy redshift survey on the basis of a conventional estimator for $\beta_0$ which neglects both the geometrical distortion effect and the time evolution of the parameter $\beta(z)$. If the magnitude limit of the survey is as faint as 18.5 (in B-band) as in the case of the Sloan Digital Sky Survey, the systematic error ranges between -20% and 10% depending on the cosmological parameters. Although such systematic errors are smaller than the statistical errors in the current surveys, they will dominate the expected statistical error for future surveys.Comment: 9 pages, 5 figs, aastex, ApJ in press, replaced version includes minor correction

Designing a mobile augmented memory system for people with traumatic brain injuries

Augmented memory systems help people remember events in their lives. Individuals with Traumatic Brain Injury (TBI) often have memory impairments. We conducted a user study to learn about strategies individuals with TBI use to remember events in their lives. We explored what characteristics individuals with TBI expect of an augmented memory system. We then investigated these aspects in an initial mobile app design, and propose here a concept for a rehearsal application that addresses the issues found in our studies

The Nonlinear Cosmological Matter Power Spectrum with Massive Neutrinos I: The Halo Model

Measurements of the linear power spectrum of galaxies have placed tight constraints on neutrino masses. We extend the framework of the halo model of cosmological nonlinear matter clustering to include the effect of massive neutrino infall into cold dark matter (CDM) halos. The magnitude of the effect of neutrino clustering for three degenerate mass neutrinos with m_nu=0.9 eV is of order ~1%, within the potential sensitivity of upcoming weak lensing surveys. In order to use these measurements to further constrain--or eventually detect--neutrino masses, accurate theoretical predictions of the nonlinear power spectrum in the presence of massive neutrinos will be needed, likely only possible through high-resolution multiple particle (neutrino, CDM and baryon) simulations.Comment: v2: matches PRD versio

Redshift space correlations and scale-dependent stochastic biasing of density peaks

We calculate the redshift space correlation function and the power spectrum of density peaks of a Gaussian random field. In the linear regime k < 0.1 h/Mpc, the redshift space power spectrum is P^s_{pk}(k,u) = exp(-f^2 s_{vel}^2 k^2 u^2) * [b_{pk}(k) + b_{vel}(k) f u^2]^2 * P_m(k), where u is the angle with respect to the line of sight, s_{vel} is the one-dimensional velocity dispersion, f is the growth rate, and b_{pk}(k) and b_{vel}(k) are k-dependent linear spatial and velocity bias factors. For peaks, the value of s_{vel} depends upon the functional form of b_{vel}. The peaks model is remarkable because it has unbiased velocities -- peak motions are driven by dark matter flows -- but, in order to achieve this, b_{vel} is k-dependent. We speculate that this is true in general: k-dependence of the spatial bias will lead to k-dependence of b_{vel} even if the biased tracers flow with the dark matter. Because of the k-dependence of the linear bias parameters, standard manipulations applied to the peak model will lead to k-dependent estimates of the growth factor that could erroneously be interpreted as a signature of modified dark energy or gravity. We use the Fisher formalism to show that the constraint on the growth rate f is degraded by a factor of two if one allows for a k-dependent velocity bias of the peak type. We discuss a simple estimate of nonlinear evolution and illustrate the effect of the peak bias on the redshift space multipoles. For k < 0.1 h/Mpc, the peak bias is deterministic but k-dependent, so the configuration space bias is stochastic and scale dependent, both in real and redshift space. We provide expressions for this stochasticity and its evolution (abridged).Comment: 24 pages, 7 figures (v3): references added (v4): added figure+appendix. In press in PR

The Mass Power Spectrum in Quintessence Cosmological Models

We present simple analytic approximations for the linear and fully evolved nonlinear mass power spectrum for spatially flat cold dark matter (CDM) cosmological models with quintessence (Q). Quintessence is a time evolving, spatially inhomogeneous energy component with negative pressure and an equation of state w_Q < 0. It clusters gravitationally on large length scales but remains smooth like the cosmological constant on small length scales. We show that the clustering scale is determined by the Compton wavelength of the Q-field and derive a shape parameter, \Gamma_Q, to characterize the linear mass power spectrum. The growth of linear perturbations as functions of redshift, w_Q, and matter density \Omega_m is also quantified. Calibrating to N-body simulations, we construct a simple extension of the formula by Ma (1998) that closely approximates the nonlinear power spectrum for a range of plausible QCDM models.Comment: 5 pages with 3 inserted postscript figures, AAS LaTeX v4.0 emulateapj.sty. Astrophysical Journal Letters, in pres

The EXOSAT medium-energy slew survey catalog

We present a catalog of X-ray sources observed during slew maneuvers by the Medium Energy Detector Array onboard the EXOSAT Observatory. The EXOSAT Medium Energy slew-survey catalog (EXMS) provides a unique record of the 1--8 keV X-ray sky between 1983 and 1986. 98% of the sky was observed, with 85% receiving an exposure of >60 s. 1210 sources were detected. By comparing these source positions with other catalogs, identifications are given for 992 detections (82% of the sample). These identifications consist of 250 distinct objects, including 95 different X-ray binary systems, and 14 different AGN. A further 58 detections have multiple candidates, while 160 detections remain unidentified. Collimator transmission corrected 1-8 keV count rates are given for the identified sources, together with raw count rates for the other detections. The construction of the EXMS and the checks performed to ensure the validity of the derived source properties are discussed. A publically available version of this catalog is maintained on the EXOSAT database and archive system (telnet://[email protected]).Comment: 52 pages. 22 Figures. To be published in A&AS. For more information, see http://astro.estec.esa.nl/SA-general/Projects/Exosat/exmsintro.htm

Cosmological Model Predictions for Weak Lensing: Linear and Nonlinear Regimes

Weak lensing by large scale structure induces correlated ellipticities in the images of distant galaxies. The two-point correlation is determined by the matter power spectrum along the line of sight. We use the fully nonlinear evolution of the power spectrum to compute the predicted ellipticity correlation. We present results for different measures of the second moment for angular scales \theta \simeq 1'-3 degrees and for alternative normalizations of the power spectrum, in order to explore the best strategy for constraining the cosmological parameters. Normalizing to observed cluster abundance the rms amplitude of ellipticity within a 15' radius is \simeq 0.01 z_s^{0.6}, almost independent of the cosmological model, with z_s being the median redshift of background galaxies. Nonlinear effects in the evolution of the power spectrum significantly enhance the ellipticity for \theta < 10' -- on 1' the rms ellipticity is \simeq 0.05, which is nearly twice the linear prediction. This enhancement means that the signal to noise for the ellipticity is only weakly increasing with angle for 2'< \theta < 2 degrees, unlike the expectation from linear theory that it is strongly peaked on degree scales. The scaling with cosmological parameters also changes due to nonlinear effects. By measuring the correlations on small (nonlinear) and large (linear) angular scales, different cosmological parameters can be independently constrained to obtain a model independent estimate of both power spectrum amplitude and matter density \Omega_m. Nonlinear effects also modify the probability distribution of the ellipticity. Using second order perturbation theory we find that over most of the range of interest there are significant deviations from a normal distribution.Comment: 38 pages, 11 figures included. Extended discussion of observational prospects, matches accepted version to appear in Ap

Serological diagnosis of Q fever endocarditis

The diagnosis of Q fever endocarditis cannot be made by bacterial cultures and necessitates serological identification of specific antibodies to Coxiella burnetii which stimulates mainly the production of anti-phase II antibodies during the acute diséase, but primarily anti-phase I antibodies in endocarditis. Indirect micro-immunofluorescence allows rapid detection of specific IgA, IgG and IgM. The results of serological analyses of 191 acute cases of Q fever were compared with those of 8 cases of Coxiella burnetii endocarditis. All sera were evaluated by complement fixation and microimmunofluorescence tests. The highest titre differences between primary Q fever and Q fever endocarditis were observed with anti-phase IIgA and IgG antibodies measured by microimmunofluorescence followed by anti-phase I antibodies measured by complement fixation tests. Anti-phase IIgG and IgM titres were consistently higher than anti-phase II titres in endocarditis. The reverse is true in acute Q fever. In addition, anti-phase I Ig A appeared to be diagnostic for Coxiella burnetii endocarditis. Accordingly we recommend the testing of these specific IgA, IgG, and IgM by microimmunofluorescence in cases of culture-negative endocarditis. These tests could also prove useful for following the development of Coxiella burnetii endocarditis in patients under treatmen