10 research outputs found
Dark energy constraints from cosmic shear power spectra: impact of intrinsic alignments on photometric redshift requirements
Cosmic shear constrains cosmology by exploiting the apparent alignments of
pairs of galaxies due to gravitational lensing by intervening mass clumps.
However galaxies may become (intrinsically) aligned with each other, and with
nearby mass clumps, during their formation. This effect needs to be
disentangled from the cosmic shear signal to place constraints on cosmology. We
use the linear intrinsic alignment model as a base and compare it to an
alternative model and data. If intrinsic alignments are ignored then the dark
energy equation of state is biased by ~50 per cent. We examine how the number
of tomographic redshift bins affects uncertainties on cosmological parameters
and find that when intrinsic alignments are included two or more times as many
bins are required to obtain 80 per cent of the available information. We
investigate how the degradation in the dark energy figure of merit depends on
the photometric redshift scatter. Previous studies have shown that lensing does
not place stringent requirements on the photometric redshift uncertainty, so
long as the uncertainty is well known. However, if intrinsic alignments are
included the requirements become a factor of three tighter. These results are
quite insensitive to the fraction of catastrophic outliers, assuming that this
fraction is well known. We show the effect of uncertainties in photometric
redshift bias and scatter. Finally we quantify how priors on the intrinsic
alignment model would improve dark energy constraints.Comment: 14 pages and 9 figures. Replaced with final version accepted in
"Gravitational Lensing" Focus Issue of the New Journal of Physics at
http://www.iop.org/EJ/abstract/1367-2630/9/12/E0
Gravitational lensing due to dark matter modelled by vector field
The specified constant 4-vector field reproducing the spherically symmetric
stationary metric of cold dark matter halo in the region of flat rotation
curves results in a constant angle of light deflection at small impact
distances. The effective deflecting mass is factor greater than the
dark matter mass. The perturbation of deflection picture due to the halo edge
is evaluated.Comment: 17 pages, LaTeX iopart class, 10 eps figures; explanaitions and
discussion are extended and improved, reference added; version to appear in
Classical and Quantum Gravit
Transients from initial conditions based on Lagrangian perturbation theory in N-body simulations
We explore the initial conditions for cosmological N-body simulations
suitable for calculating the skewness and kurtosis of the density field. In
general, the initial conditions based on the perturbation theory (PT) provide
incorrect second-order and higher-order growth. These errors implied by the use
of the perturbation theory to set up the initial conditions in N-body
simulations are called transients. Unless these transients are completely
suppressed compared with the dominant growing mode, we can not reproduce the
correct evolution of cumulants with orders higher than two, even though there
is no problem with the numerical scheme. We investigate the impact of
transients on the observable statistical quantities by performing -body
simulations with initial conditions based on Lagrangian perturbation theory
(LPT). We show that the effects of transients on the kurtosis from the initial
conditions, based on second-order Lagrangian perturbation theory (2LPT) have
almost disappeared by , as long as the initial conditions are set at . This means that for practical purposes, the initial conditions based on
2LPT are accurate enough for numerical calculations of skewness and kurtosis.Comment: 21 pages, 5 figures; accepted for publication in JCA
Probing dark energy with cluster counts and cosmic shear power spectra: including the full covariance
(Abridged) Combining cosmic shear power spectra and cluster counts is
powerful to improve cosmological parameter constraints and/or test inherent
systematics. However they probe the same cosmic mass density field, if the two
are drawn from the same survey region, and therefore the combination may be
less powerful than first thought. We investigate the cross-covariance between
the cosmic shear power spectra and the cluster counts based on the halo model
approach, where the cross-covariance arises from the three-point correlations
of the underlying mass density field. Fully taking into account the
cross-covariance as well as non-Gaussian errors on the lensing power spectrum
covariance, we find a significant cross-correlation between the lensing power
spectrum signals at multipoles l~10^3 and the cluster counts containing halos
with masses M>10^{14}Msun. Including the cross-covariance for the combined
measurement degrades and in some cases improves the total signal-to-noise
ratios up to plus or minus 20% relative to when the two are independent. For
cosmological parameter determination, the cross-covariance has a smaller effect
as a result of working in a multi-dimensional parameter space, implying that
the two observables can be considered independent to a good approximation. We
also discuss that cluster count experiments using lensing-selected mass peaks
could be more complementary to cosmic shear tomography than mass-selected
cluster counts of the corresponding mass threshold. Using lensing selected
clusters with a realistic usable detection threshold (S/N~6 for a ground-based
survey), the uncertainty on each dark energy parameter may be roughly halved by
the combined experiments, relative to using the power spectra alone.Comment: 32 pages, 15 figures. Revised version, invited original contribution
to gravitational lensing focus issue, New Journal of Physic
Non-Gaussianity of the density distribution in accelerating universes II:N-body simulations
We explore the possibility of putting constraints on dark energy models with
statistical property of large scale structure in the non-linear region. In
particular, we investigate the dependence of non-Gaussianity of the
smoothed density distribution generated by the nonlinear dynamics. In order to
follow the non-linear evolution of the density fluctuations, we apply N-body
simulations based on scheme. We show that the relative difference
between non-Gaussianity of model and that of model is (skewness) and (kurtosis) for Mpc. We also calculate the
correspondent quantities for Mpc, (skewness) and
(kurtosis), and the difference turn out to be greater, even though
non-linearity in this scale is so strong that the complex physical processes
about galaxy formation affect the galaxy distribution. From this, we can expect
that the difference can be tested by all sky galaxy surveys with the help of
mock catalogs created by selection functions, which suggests that
non-Gaussianity of the density distribution potentially plays an important role
for extracting information on dark energy.Comment: 21 pages, 14 figure
Cluster Lenses
Clusters of galaxies are the most recently assembled, massive, bound
structures in the Universe. As predicted by General Relativity, given their
masses, clusters strongly deform space-time in their vicinity. Clusters act as
some of the most powerful gravitational lenses in the Universe. Light rays
traversing through clusters from distant sources are hence deflected, and the
resulting images of these distant objects therefore appear distorted and
magnified. Lensing by clusters occurs in two regimes, each with unique
observational signatures. The strong lensing regime is characterized by effects
readily seen by eye, namely, the production of giant arcs, multiple-images, and
arclets. The weak lensing regime is characterized by small deformations in the
shapes of background galaxies only detectable statistically. Cluster lenses
have been exploited successfully to address several important current questions
in cosmology: (i) the study of the lens(es) - understanding cluster mass
distributions and issues pertaining to cluster formation and evolution, as well
as constraining the nature of dark matter; (ii) the study of the lensed objects
- probing the properties of the background lensed galaxy population - which is
statistically at higher redshifts and of lower intrinsic luminosity thus
enabling the probing of galaxy formation at the earliest times right up to the
Dark Ages; and (iii) the study of the geometry of the Universe - as the
strength of lensing depends on the ratios of angular diameter distances between
the lens, source and observer, lens deflections are sensitive to the value of
cosmological parameters and offer a powerful geometric tool to probe Dark
Energy. In this review, we present the basics of cluster lensing and provide a
current status report of the field.Comment: About 120 pages - Published in Open Access at:
http://www.springerlink.com/content/j183018170485723/ . arXiv admin note:
text overlap with arXiv:astro-ph/0504478 and arXiv:1003.3674 by other author
Gravitational Lensing
Gravitational lensing has developed into one of the most powerful tools for
the analysis of the dark universe. This review summarises the theory of
gravitational lensing, its main current applications and representative results
achieved so far. It has two parts. In the first, starting from the equation of
geodesic deviation, the equations of thin and extended gravitational lensing
are derived. In the second, gravitational lensing by stars and planets,
galaxies, galaxy clusters and large-scale structures is discussed and
summarised.Comment: Invited review article to appear in Classical and Quantum Gravity, 85
pages, 15 figure
Gravitational Lensing in Astronomy
Deflection of light by gravity was predicted by General Relativity and
observationaly confirmed in 1919. In the following decades various aspects of
the gravitational lens effect were explored theoretically, among them the
possibility of multiple or ring-like images of background sources, the use of
lensing as a gravitational telescope on very faint and distant objects, and the
possibility to determine Hubble's constant with lensing. Only relatively
recently gravitational lensing became an observational science after the
discovery of the first doubly imaged quasar in 1979. Today lensing is a booming
part of astrophysics.
In addition to multiply-imaged quasars, a number of other aspects of lensing
have been discovered since, e.g. giant luminous arcs, quasar microlensing,
Einstein rings, galactic microlensing events, arclets, or weak gravitational
lensing. By now literally hundreds of individual gravitational lens phenomena
are known.
Although still in its childhood, lensing has established itself as a very
useful astrophysical tool with some remarkable successes. It has contributed
significant new results in areas as different as the cosmological distance
scale, the large scale matter distribution in the universe, mass and mass
distribution of galaxy clusters, physics of quasars, dark matter in galaxy
halos, or galaxy structure.Comment: Review article for "Living Reviews in Relativity", see
http://www.livingreviews.org . 41 pages, latex, 22 figures (partly in GIF
format due to size constraints). High quality postscript files can be
obtained electronically at http://www.aip.de:8080/~jkw/review_figures.htm