862 research outputs found
The Nonlinear Redshift Space Power Spectrum: Omega from Redshift Surveys
We examine the anisotropies in the power spectrum by the mapping of real to
redshift space. Using the Zel'dovich approximation, we obtain an analytic
expression for the nonlinear redshift space power spectrum in the distant
observer limit. For a given unbiased galaxy distribution in redshift space, the
anisotropies in the power spectrum depend on the parameter , where is the density parameter. We quantify these
anisotropies by the ratio, , of the quadrupole to monopole angular moments
of the power spectrum. In contrast to linear theory, the Zel'dovich
approximation predicts a decline in with decreasing scale. This departure
from linear theory is due to nonlinear dynamics and not a result of incoherent
random velocities. The rate of decline depends strongly on and the
initial power spectrum. However, we find a {\it universal} relation between the
quantity (where the linear theory value of ) and the
dimensionless variable , where is a wavenumber determined by
the scale of nonlinear structures. The universal relation is in good agreement
with a large N-body simulation. This universal relation greatly extends the
scales over which redshift distortions can be used as a probe of . A
preliminary application to the 1.2 Jy IRAS yields if IRAS
galaxies are unbiased.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Radiative Transfer Effect on Ultraviolet Pumping of the 21cm Line in the High Redshift Universe
During the epoch of reionization the 21cm signal is sensitive to the
scattering rate of the ultraviolet photons, redshifting across the Lyman_alpha
resonance. Here we calculate the photon scattering rate profile for a single
ultraviolet source. After taking into account previously neglected natural
broadening of the resonance line, we find that photons approach the resonance
frequency and experience most scatterings at a significantly smaller distance
from the source than naively expected r=(dnu/nu_0)(c/H), where dnu=nu-nu_0 is
the initial frequency offset, and the discrepancy increases as the initial
frequency offset decreases. As a consequence, the scattering rate P(r) drops
much faster with increasing distance than the previously assumed 1/r^2 profile.
Near the source (r<1Mpc comoving), the scattering rate of photons that redshift
into the Ly_alpha resonance converges to P(r) \propto r^{-7/3}. The scattering
rate of Ly_alpha photons produced by splitting of photons that redshift into a
higher resonance (Ly_gamma, Ly_delta, etc.) is only weakly affected by the
radiative transfer, while the sum of scattering rates of Ly_alpha photons
produced from all higher resonances also converges to P(r) \propto r^{-7/3}
near the source. At 15<z<35, on scales of ~0.01-20Mpc/h (comoving), the total
scattering rate of Ly_alpha photons from all Lyman resonances is found to be
higher by a factor of ~1+0.3[(1+z)/20]^{2/3} than obtained without full
radiative transfer. Consequently, during the early stage of reionization, the
differential brightness of 21cm signal against the cosmic microwave background
is also boosted by a similar factor.Comment: 7 pages, 4 figures, submitted to Ap
The impact of galactic winds from LBGs on the Intergalactic Medium
An excess of sight-lines close to Lyman-break galaxies (LBGs) with little or
no absorption in QSO absorption spectra has been reported and has been
interpreted as the effect of galactic winds on the Intergalactic Medium. We use
here numerical simulations to investigate the flux probability function close
to plausible sites of LBGs. We show that the flux distribution near our LBGs in
the simulation depends strongly on redshift, and is very sensitive to the
averaging procedure. We show that a model without galactic winds and a model
with a wind bubble size of 0.5Mpc/h (comoving) are equally consistent with the
new determination of the conditional flux distribution by Adelberger et al.
(2005). Models with the larger bubble sizes suggested by the previous
observations of Adelberger et al. (2003) based on a much smaller sample at
higher redshift are not consistent with the new data. We therefore argue that
the volume filling factor of galactic winds driven by LBGs may be much smaller
than previously thought, and that most of the metals responsible for the metal
absorption associated with the low column density Lya forest are unlikely to
have been ejected by LBGs.Comment: 5 pages, 3 figure
A direct probe of cosmological power spectra of the peculiar velocity field and the gravitational lensing magnification from photometric redshift surveys
The cosmological peculiar velocity field (deviations from the pure Hubble
flow) of matter carries significant information on dark energy, dark matter and
the underlying theory of gravity on large scales. Peculiar motions of galaxies
introduce systematic deviations between the observed galaxy redshifts z and the
corresponding cosmological redshifts z_cos. A novel method for estimating the
angular power spectrum of the peculiar velocity field based on observations of
galaxy redshifts and apparent magnitudes m (or equivalently fluxes) is
presented. This method exploits the fact that a mean relation between z_cos and
m of galaxies can be derived from all galaxies in a redshift-magnitude survey.
Given a galaxy magnitude, it is shown that the z_cos(m) relation yields its
cosmological redshift with a 1-sigma error of sigma_z~0.3 for a survey like
Euclid (~10^9 galaxies at z<~2), and can be used to constrain the angular power
spectrum of z-z_cos(m) with a high signal-to-noise ratio. At large angular
separations corresponding to l<~15, we obtain significant constraints on the
power spectrum of the peculiar velocity field. At 15<~l<~60, magnitude shifts
in the z_cos(m) relation caused by gravitational lensing magnification
dominate, allowing us to probe the line-of-sight integral of the gravitational
potential. Effects related to the environmental dependence in the luminosity
function can easily be computed and their contamination removed from the
estimated power spectra. The amplitude of the combined velocity and lensing
power spectra at z~1 can be measured with <~5% accuracy.Comment: 22 pages, 3 figures; added a discussion of systematic errors,
accepted for publication in JCA
Neonatal sepsis syndrome (early onset)
This issue of eMedRef provides information to clinicians on the pathophysiology, diagnosis, and therapeutics of early onset neonatal sepsis syndrome
Patchy He II reionization and the physical state of the IGM
We present a Monte-Carlo model of He II reionization by QSOs and its effect
on the thermal state of the clumpy intergalactic medium (IGM). The model
assumes that patchy reionization develops as a result of the discrete
distribution of QSOs. It includes various recipes for the propagation of the
ionizing photons, and treats photo-heating self-consistently. The model
provides the fraction of He III, the mean temperature in the IGM, and the He II
mean optical depth -- all as a function of redshift. It also predicts the
evolution of the local temperature versus density relation during reionization.
Our findings are as follows: The fraction of He III increases gradually until
it becomes close to unity at . The He II mean optical depth
decreases from at to at .
The mean temperature rises gradually between and and
declines slowly at lower redshifts. The model predicts a flattening of the
temperature-density relation with significant increase in the scatter during
reionization at . Towards the end of reionization the scatter is
reduced and a tight relation is re-established. This scatter should be
incorporated in the analysis of the Ly forest at . Comparison
with observational results of the optical depth and the mean temperature at
moderate redshifts constrains several key physical parameters.Comment: 18 pages, 9 figures; Changed content. Accepted for publication in
MNRA
Translating {Hausdorff} Is Hard: {F}ine-Grained Lower Bounds for {Hausdorff} Distance Under Translation
Polygon Placement Revisited: (Degree of Freedom + 1)-SUM Hardness and an Improvement via Offline Dynamic Rectangle Union
We revisit the classical problem of determining the largest copy of a simple polygon that can be placed into a simple polygon . Despite significant effort, known algorithms require high polynomial running times. (Barequet and Har-Peled, 2001) give a lower bound of under the 3SUM conjecture when and are (convex) polygons with vertices each. This leaves open whether we can establish (1) hardness beyond quadratic time and (2) any superlinear bound for constant-sized or . In this paper, we affirmatively answer these questions under the SUM conjecture, proving natural hardness results that increase with each degree of freedom (scaling, -translation, -translation, rotation): (1) Finding the largest copy of that can be -translated into requires time under the 3SUM conjecture. (2) Finding the largest copy of that can be arbitrarily translated into requires time under the 4SUM conjecture. (3) The above lower bounds are almost tight when one of the polygons is of constant size: we obtain an -time algorithm for orthogonal polygons with and vertices, respectively. (4) Finding the largest copy of that can be arbitrarily rotated and translated into requires time under the 5SUM conjecture. We are not aware of any other such natural degree of freedom -SUM hardness for a geometric optimization problem
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