5,323 research outputs found
Parameter likelihood of intrinsic ellipticity correlations
Subject of this paper are the statistical properties of ellipticity
alignments between galaxies evoked by their coupled angular momenta. Starting
from physical angular momentum models, we bridge the gap towards ellipticity
correlations, ellipticity spectra and derived quantities such as aperture
moments, comparing the intrinsic signals with those generated by gravitational
lensing, with the projected galaxy sample of EUCLID in mind. We investigate the
dependence of intrinsic ellipticity correlations on cosmological parameters and
show that intrinsic ellipticity correlations give rise to non-Gaussian
likelihoods as a result of nonlinear functional dependencies. Comparing
intrinsic ellipticity spectra to weak lensing spectra we quantify the magnitude
of their contaminating effect on the estimation of cosmological parameters and
find that biases on dark energy parameters are very small in an
angular-momentum based model in contrast to the linear alignment model commonly
used. Finally, we quantify whether intrinsic ellipticities can be measured in
the presence of the much stronger weak lensing induced ellipticity
correlations, if prior knowledge on a cosmological model is assumed.Comment: 14 pages, 8 figures, submitted to MNRA
Probing the cosmic web: inter-cluster filament detection using gravitational lensing
The problem of detecting dark matter filaments in the cosmic web is
considered. Weak lensing is an ideal probe of dark matter, and therefore forms
the basis of particularly promising detection methods. We consider and develop
a number of weak lensing techniques that could be used to detect filaments in
individual or stacked cluster fields, and apply them to synthetic lensing data
sets in the fields of clusters from the Millennium Simulation. These techniques
are multipole moments of the shear and convergence, mass reconstruction, and
parameterized fits to filament mass profiles using a Markov Chain Monte Carlo
approach. In particular, two new filament detection techniques are explored
(multipole shear filters and Markov Chain Monte Carlo mass profile fits), and
we outline the quality of data required to be able to identify and quantify
filament profiles. We also consider the effects of large scale structure on
filament detection. We conclude that using these techniques, there will be
realistic prospects of detecting filaments in data from future space-based
missions. The methods presented in this paper will be of great use in the
identification of dark matter filaments in future surveys.Comment: 12 pages, 4 figures, MNRAS accepted, (replacement due to corrupted
end of pdf file
Constraining Cosmology with High Convergence Regions in Weak Lensing Surveys
We propose to use a simple observable, the fractional area of "hot spots" in
weak gravitational lensing mass maps which are detected with high significance,
to determine background cosmological parameters. Because these high-convergence
regions are directly related to the physical nonlinear structures of the
universe, they derive cosmological information mainly from the nonlinear regime
of density fluctuations. We show that in combination with future cosmic
microwave background anisotropy measurements, this method can place constraints
on cosmological parameters that are comparable to those from the redshift
distribution of galaxy cluster abundances. The main advantage of the statistic
proposed in this paper is that projection effects, normally the main source of
uncertainty when determining the presence and the mass of a galaxy cluster,
here serve as a source of information.Comment: 14 pages, 4 figures, accepted for publication in Astrophysical
Journa
Lyman-alpha emission galaxies at a redshift of z = 5.7 in the FORS Deep Field
We present the results of a search for Lyman-alpha emission galaxies at z~
5.7 in the FORS Deep Field. The objective of this study is to improve the faint
end of the luminosity function of high-redshift Lyman-alpha emitting galaxies
and to derive properties of intrinsically faint Lyman-alpha emission galaxies
in the young universe. Using FORS2 at the ESO VLT and a set of special
interference filters, we identified candidates for high-redshift Lyman-alpha
galaxies. We then used FORS2 in spectroscopic mode to verify the
identifications and to study their spectral properties. The narrow-band
photometry resulted in the detection of 15 likely Lyman-alpha emission
galaxies. Spectra with an adequate exposure time could be obtained for eight
galaxies. In all these cases the presence of Lyman-alpha emission at z = 5.7
was confirmed spectroscopically. The line fluxes of the 15 candidates range
between 3 and 16 * 10^-21 Wm^-2, which corresponds to star-formation rates not
corrected for dust between 1 and 5 Msun/yr. The luminosity function derived for
our photometrically identified objects extends the published luminosity
functions of intrinsically brighter Lyman-alpha galaxies. With this technique
the study of high-redshift Lyman-alpha emission galaxies can be extended to low
intrinsic luminosities.Comment: 9 pages, 17 figures. Accepted by A&A. PDF version with higher
resolution figures here:
http://www.lsw.uni-heidelberg.de/users/jheidt/fdf/pubs/fdflae5_7_110406.pd
On the validity of the Born approximation for weak cosmic flexions
Weak lensing calculations are often made under the assumption of the Born
approximation, where the ray path is approximated as a straight radial line. In
addition, lens-lens couplings where there are several deflections along the
light ray are often neglected. We examine the effect of dropping the Born
approximation and taking lens-lens couplings into account, for weak lensing
effects up to second order (cosmic flexion), by making a perturbative expansion
in the light path. We present a diagrammatic representation of the resulting
corrections to the lensing effects. The flexion signal, which measures the
derivative of the density field, acquires correction terms proportional to the
squared gravitational shear; we also find that by dropping the Born
approximation, two further degrees of freedom of the lensing distortion can be
excited (the twist components), in addition to the four standard flexion
components. We derive angular power spectra of the flexion and twist, with and
without the Born-approximation and lens-lens couplings and confirm that the
Born approximation is an excellent approximation for weak cosmic flexions,
except at very small scales.Comment: 12 pages, 5 figures, submitted to MNRA
Optimal capture of non-Gaussianity in weak lensing surveys: power spectrum, bispectrum and halo counts
We compare the efficiency of weak lensing-selected galaxy clusters counts and
of the weak lensing bispectrum at capturing non-Gaussian features in the dark
matter distribution. We use the halo model to compute the weak lensing power
spectrum, the bispectrum and the expected number of detected clusters, and
derive constraints on cosmological parameters for a large, low systematic weak
lensing survey, by focusing on the - plane and on the dark
energy equation of state. We separate the power spectrum into the resolved and
the unresolved parts of the data, the resolved part being defined as detected
clusters, and the unresolved part as the rest of the field. We consider four
kinds of clusters counts, taking into account different amount of information :
signal-to-noise ratio peak counts; counts as a function of clusters' mass;
counts as a function of clusters' redshift; and counts as a function of
clusters' mass and redshift. We show that when combined with the power
spectrum, those four kinds of counts provide similar constraints, thus allowing
one to perform the most direct counts, signal-to-noise peaks counts, and get
percent level constraints on cosmological parameters. We show that the weak
lensing bispectrum gives constraints comparable to those given by the power
spectrum and captures non-Gaussian features as well as clusters counts, its
combination with the power spectrum giving errors on cosmological parameters
that are similar to, if not marginally smaller than, those obtained when
combining the power spectrum with cluster counts. We finally note that in order
to reach its potential, the weak lensing bispectrum must be computed using all
triangle configurations, as equilateral triangles alone do not provide useful
information.Comment: Matches ApJ-accepted versio
A Predictor-Informed Multi-Subject Bayesian Approach for Dynamic Functional Connectivity
Time Varying Functional Connectivity (TVFC) investigates how the interactions
among brain regions vary over the course of an fMRI experiment. The transitions
between different individual connectivity states can be modulated by changes in
underlying physiological mechanisms that drive functional network dynamics,
e.g., changes in attention or cognitive effort as measured by pupil dilation.
In this paper, we develop a multi-subject Bayesian framework for estimating
dynamic functional networks as a function of time-varying exogenous
physiological covariates that are simultaneously recorded in each subject
during the fMRI experiment. More specifically, we consider a dynamic Gaussian
graphical model approach, where a non-homogeneous hidden Markov model is
employed to classify the fMRI time series into latent neurological states,
borrowing strength over the entire time course of the experiment. The
state-transition probabilities are assumed to vary over time and across
subjects, as a function of the underlying covariates, allowing for the
estimation of recurrent connectivity patterns and the sharing of networks among
the subjects. Our modeling approach further assumes sparsity in the network
structures, via shrinkage priors. We achieve edge selection in the estimated
graph structures, by introducing a multi-comparison procedure for
shrinkage-based inferences with Bayesian false discovery rate control. We apply
our modeling framework on a resting-state experiment where fMRI data have been
collected concurrently with pupillometry measurements, leading us to assess the
heterogeneity of the effects of changes in pupil dilation, previously linked to
changes in norepinephrine-containing locus coeruleus, on the subjects'
propensity to change connectivity states
The FORS Deep Field Spectroscopic Survey
We present a catalogue and atlas of low-resolution spectra of a well defined
sample of 341 objects in the FORS Deep Field. All spectra were obtained with
the FORS instruments at the ESO VLT with essentially the same spectroscopic
set-up. The observed extragalactic objects cover the redshift range 0.1 to 5.0.
98 objects are starburst galaxies and QSOs at z > 2. Using this data set we
investigated the evolution of the characteristic spectral properties of bright
starburst galaxies and their mutual relations as a function of the redshift.
Significant evolutionary effects were found for redshifts 2 < z < 4. Most
conspicuous are the increase of the average C IV absorption strength, of the
dust reddening, and of the intrinsic UV luminosity, and the decrease of the
average Ly alpha emission strength with decreasing redshift. In part the
observed evolutionary effects can be attributed to an increase of the
metallicity of the galaxies with cosmic age. Moreover, the increase of the
total star-formation rates and the stronger obscuration of the starburst cores
by dusty gas clouds suggest the occurrence of more massive starbursts at later
cosmic epochs.Comment: 24 pages, 25 figures (35 PS files), 4 tables, accepted for
publication in A&A. v2: minor typos corrected and references update
Simultaneous measurement of cosmology and intrinsic alignments using joint cosmic shear and galaxy number density correlations
Cosmic shear is a powerful method to constrain cosmology, provided that any
systematic effects are under control. The intrinsic alignment of galaxies is
expected to severely bias parameter estimates if not taken into account. We
explore the potential of a joint analysis of tomographic galaxy ellipticity,
galaxy number density, and ellipticity-number density cross-correlations to
simultaneously constrain cosmology and self-calibrate unknown intrinsic
alignment and galaxy bias contributions. We treat intrinsic alignments and
galaxy biasing as free functions of scale and redshift and marginalise over the
resulting parameter sets. Constraints on cosmology are calculated by combining
the likelihoods from all two-point correlations between galaxy ellipticity and
galaxy number density. The information required for these calculations is
already available in a standard cosmic shear dataset. We include contributions
to these functions from cosmic shear, intrinsic alignments, galaxy clustering
and magnification effects. In a Fisher matrix analysis we compare our
constraints with those from cosmic shear alone in the absence of intrinsic
alignments. For a potential future large area survey, such as Euclid, the extra
information from the additional correlation functions can make up for the
additional free parameters in the intrinsic alignment and galaxy bias terms,
depending on the flexibility in the models. For example, the Dark Energy Task
Force figure of merit is recovered even when more than 100 free parameters are
marginalised over. We find that the redshift quality requirements are similar
to those calculated in the absence of intrinsic alignments.Comment: 22 pages, 10 figures; extended discussion, otherwise minor changes to
match accepted version; published in Astronomy and Astrophysic
Detection of Weak Gravitational Lensing by Large-scale Structure
We report a detection of the coherent distortion of faint galaxies arising
from gravitational lensing by foreground structures. This ``cosmic shear'' is
potentially the most direct measure of the mass power spectrum, as it is
unaffected by poorly-justified assumptions made concerning the biasing of the
distribution. Our detection is based on an initial imaging study of 14
separated 8' x 16' fields observed in good, homogeneous conditions with the
prime focus EEV CCD camera of the 4.2m William Herschel Telescope. We detect an
rms shear of 1.6% in 8' x 8' cells, with a significance of 3.4 sigma. We
carefully justify this detection by quantifying various systematic effects and
carrying out extensive simulations of the recovery of the shear signal from
artificial images defined according to measured instrument characteristics. We
also verify our detection by computing the cross-correlation between the shear
in adjacent cells. Including (gaussian) cosmic variance, we measure the shear
variance to be (0.016)^2 plus/minus (0.012)^2 plus/minus (0.006)^2, where these
1 sigma errors correspond to statistical and systematic uncertainties,
respectively. Our measurements are consistent with the predictions of
cluster-normalised CDM models (within 1 sigma) but a COBE-normalised SCDM model
is ruled out at the 3.0 sigma level. For the currently-favoured Lambda-CDM
model (with Omega_m = 0.3), our measurement provides a normalisation of the
mass power spectrum of sigma_8 = 1.5 plus/minus 0.5, fully consistent with that
derived from cluster abundances. Our result demonstrates that ground-based
telescopes can, with adequate care, be used to constrain the mass power
spectrum on various scales. The present results are limited mainly by cosmic
variance, which can be overcome in the near future with more observations.Comment: 17 LaTex pages, including 13 figures and 3 tables. Accepted for
publication in MNRAS, minor revisio
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