2,060 research outputs found
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 , and obtain analytic formulae to infer the
deceleration parameter , the density parameter and the
derivative of the bias parameter at 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 from a galaxy
redshift survey on the basis of a conventional estimator for which
neglects both the geometrical distortion effect and the time evolution of the
parameter . 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
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