1,496 research outputs found
Cosmic Shear from Galaxy Spins
We discuss the origin of galactic angular momentum, and the statistics of the
present day spin distribution. It is expected that the galaxy spin axes are
correlated with the intermediate principal axis of the gravitational shear
tensor. This allows one to reconstruct the shear field and thereby the full
gravitational potential from the observed galaxy spin fields. We use the
direction of the angular momentum vector without any information of its
magnitude, which requires a measurement of the position angle and inclination
on the sky of each disk galaxy. We present the maximum likelihood shear
inversion procedure, which involves a constrained linear minimization. The
theory is tested against numerical simulations. We find the correlation
strength of nonlinear structures with the initial shear field, and show that
accurate large scale density reconstructions are possible at the expected noise
level.Comment: Accepted by the ApJL, revised discussion, minor changes, LaTex file,
8 pages, 1 ps figur
The Nonlinear Evolution of Galaxy Intrinsic Alignments
The non-Gaussian contribution to the intrinsic halo spin alignments is
analytically modeled and numerically detected. Assuming that the growth of
non-Gaussianity in the density fluctuations caused the tidal field to have
nonlinear-order effect on the orientations of the halo angular momentum, we
model the intrinsic halo spin alignments as a linear scaling of the density
correlations on large scales, which is different from the previous
quadratic-scaling model based on the linear tidal torque theory. Then, we
analyze the halo catalogs from the recent high-resolution Millennium Run
simulation at four different redshifts (z=0,0.5,1 and 2) and measure
quantitatively the degree of the nonlinear effect on the halo spin alignments
and its changes with redshifts. A clear signal of spin correlations is found on
scales as large as 10 Mpc/h at z=0, which marks a detection of the nonlinear
tidal effect on the intrinsic halo alignments. We also investigate how the
nonlinear effect depends on the intrinsic properties of the halos. It is found
that the degree of the nonlinear tidal effect increases as the halo mass scale
decreases, the halo specific angular momentum increases, and the halo peculiar
velocity decreases. We discuss implication of our result on the weak
gravitational lensing.Comment: ApJ in press, revised version, mistakes and typos corrected,
discussion improved, 29 pages, 11 figure
Power Spectra in Global Defect Theories of Cosmic Structure Formation
An efficient technique for computing perturbation power spectra in field
ordering theories of cosmic structure formation is introduced, enabling
computations to be carried out with unprecedented precision. Large scale
simulations are used to measure unequal time correlators of the source stress
energy, taking advantage of scaling during matter and radiation domination, and
causality, to make optimal use of the available dynamic range. The correlators
are then re-expressed in terms of a sum of eigenvector products, a
representation which we argue is optimal, enabling the computation of the final
power spectra to be performed at high accuracy. Microwave anisotropy and matter
perturbation power spectra for global strings, monopoles, textures and
non-topological textures are presented and compared with recent observations.Comment: 4 pages, compressed and uuencoded RevTex file and postscript figure
Pulsar timing arrays as imaging gravitational wave telescopes: angular resolution and source (de)confusion
Pulsar timing arrays (PTAs) will be sensitive to a finite number of
gravitational wave (GW) "point" sources (e.g. supermassive black hole
binaries). N quiet pulsars with accurately known distances d_{pulsar} can
characterize up to 2N/7 distant chirping sources per frequency bin \Delta
f_{gw}=1/T, and localize them with "diffraction limited" precision \delta\theta
\gtrsim (1/SNR)(\lambda_{gw}/d_{pulsar}). Even if the pulsar distances are
poorly known, a PTA with F frequency bins can still characterize up to
(2N/7)[1-(1/2F)] sources per bin, and the quasi-singular pattern of timing
residuals in the vicinity of a GW source still allows the source to be
localized quasi-topologically within roughly the smallest quadrilateral of
quiet pulsars that encircles it on the sky, down to a limiting resolution
\delta\theta \gtrsim (1/SNR) \sqrt{\lambda_{gw}/d_{pulsar}}. PTAs may be
unconfused, even at the lowest frequencies, with matched filtering always
appropriate.Comment: 7 pages, 1 figure, matches Phys.Rev.D versio
A Causal Source which Mimics Inflation
How unique are the inflationary predictions for the cosmic microwave
anisotropy pattern? In this paper, it is asked whether an arbitrary causal
source for perturbations in the standard hot big bang could effectively mimic
the predictions of the simplest inflationary models. A surprisingly simple
example of a `scaling' causal source is found to closely reproduce the
inflationary predictions. This letter extends the work of a previous paper
(ref. 6) to a full computation of the anisotropy pattern, including the Sachs
Wolfe integral. I speculate on the possible physics behind such a source.Comment: 4 pages, RevTex, 3 figure
Normalizing the Temperature Function of Clusters of Galaxies
We re-examine the constraints which can be robustly obtained from the
observed temperature function of X-ray cluster of galaxies. The cluster mass
function has been thoroughly studied in simulations and analytically, but a
direct simulation of the temperature function is presented here for the first
time. Adaptive hydrodynamic simulations using the cosmological Moving Mesh
Hydro code of Pen (1997a) are used to calibrate the temperature function for
different popular cosmologies. Applying the new normalizations to the
present-day cluster abundances, we find for a hyperbolic universe, and for a spatially flat universe with a cosmological constant.
The simulations followed the gravitational shock heating of the gas and dark
matter, and used a crude model for potential energy injection by supernova
heating. The error bars are dominated by uncertainties in the heating/cooling
models. We present fitting formulae for the mass-temperature conversions and
cluster abundances based on these simulations.Comment: 20 pages incl 5 figures, final version for ApJ, corrected open
universe \gamma relation, results unchange
Optimizing Observational Strategy for Future Fgas Constraints
The Planck cluster catalog is expected to contain of order a thousand galaxy
clusters, both newly discovered and previously known, detected through the
Sunyaev-Zeldovich effect over the redshift range 0 < z < 1. Follow-up X-ray
observations of a dynamically relaxed sub-sample of newly discovered Planck
clusters will improve constraints on the dark energy equation-of-state found
through measurement of the cluster gas mass fraction fgas. In view of follow-up
campaigns with XMM-Newton and Chandra, we determine the optimal redshift
distribution of a cluster sample to most tightly constrain the dark energy
equation of state. The distribution is non-trivial even for the standard w0-wa
parameterization. We then determine how much the combination of expected data
from the Planck satellite and fgas data will be able to constrain the dark
energy equation-of-state. Our analysis employs a Markov Chain Monte Carlo
method as well as a Fisher Matrix analysis. We find that these upcoming data
will be able to improve the figure-of-merit by at least a factor two.Comment: 11 pages, 8 figure
Near term measurements with 21 cm intensity mapping: neutral hydrogen fraction and BAO at z<2
It is shown that 21 cm intensity mapping could be used in the near term to
make cosmologically useful measurements. Large scale structure could be
detected using existing radio telescopes, or using prototypes for dedicated
redshift survey telescopes. This would provide a measure of the mean neutral
hydrogen density, using redshift space distortions to break the degeneracy with
the linear bias. We find that with only 200 hours of observing time on the
Green Bank Telescope, the neutral hydrogen density could be measured to 25%
precision at redshift 0.54<z<1.09. This compares favourably to current
measurements, uses independent techniques, and would settle the controversy
over an important parameter which impacts galaxy formation studies. In
addition, a 4000 hour survey would allow for the detection of baryon acoustic
oscillations, giving a cosmological distance measure at 3.5% precision. These
observation time requirements could be greatly reduced with the construction of
multiple pixel receivers. Similar results are possible using prototypes for
dedicated cylindrical telescopes on month time scales, or SKA pathfinder
aperture arrays on day time scales. Such measurements promise to improve our
understanding of these quantities while beating a path for future generations
of hydrogen surveys.Comment: 6 pages, 5 figures. Submitted to Phys. Rev. D. Addressed reviewer
comments. Changed figure format, added more detailed technical discussion,
and added forecasts for aperture arrays. Added references
Stochastic urban pluvial flood hazard maps based upon a spatial-temporal rainfall generator
It is a common practice to assign the return period of a given storm event to the urban pluvial flood event that such storm generates. However, this approach may be inappropriate as rainfall events with the same return period can produce different urban pluvial flooding events, i.e., with different associated flood extent, water levels and return periods. This depends on the characteristics of the rainfall events, such as spatial variability, and on other characteristics of the sewer system and the catchment. To address this, the paper presents an innovative contribution to produce stochastic urban pluvial flood hazard maps. A stochastic rainfall generator for urban-scale applications was employed to generate an ensemble of spatially—and temporally—variable design storms with similar return period. These were used as input to the urban drainage model of a pilot urban catchment (~9 km2) located in London, UK. Stochastic flood hazard maps were generated through a frequency analysis of the flooding generated by the various storm events. The stochastic flood hazard maps obtained show that rainfall spatial-temporal variability is an important factor in the estimation of flood likelihood in urban areas. Moreover, as compared to the flood hazard maps obtained by using a single spatially-uniform storm event, the stochastic maps generated in this study provide a more comprehensive assessment of flood hazard which enables better informed flood risk management decisions
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