155 research outputs found
Scale dependence of galaxy biasing investigated by weak gravitational lensing: An assessment using semi-analytic galaxies and simulated lensing data
Galaxies are biased tracers of the matter density on cosmological scales. For
future tests of galaxy models, we refine and assess a method to measure galaxy
biasing as function of physical scale with weak gravitational lensing. This
method enables us to reconstruct the galaxy bias factor as well as the
galaxy-matter correlation on spatial scales between for redshift-binned lens galaxies below redshift .
In the refinement, we account for an intrinsic alignment of source
ellipticities, and we correct for the magnification bias of the lens galaxies,
relevant for the galaxy-galaxy lensing signal, to improve the accuracy of the
reconstructed . For simulated data, the reconstructions achieve an
accuracy of (68\% confidence level) over the above -range for a
survey area and a typical depth of contemporary ground-based surveys.
Realistically the accuracy is, however, probably reduced to about ,
mainly by systematic uncertainties in the assumed intrinsic source alignment,
the fiducial cosmology, and the redshift distributions of lens and source
galaxies (in that order). Furthermore, our reconstruction technique employs
physical templates for and that elucidate the impact of central
galaxies and the halo-occupation statistics of satellite galaxies on the
scale-dependence of galaxy bias, which we discuss in the paper. In a first
demonstration, we apply this method to previous measurements in the
Garching-Bonn-Deep Survey and give a physical interpretation of the lens
population.Comment: 31 pages, 16 figures; corrected typos in Eqs. (31), (34), and (36
Magnification bias in the shear-ratio test: a viable mitigation strategy
Using the same lens galaxies, the ratios of tangential shears for different
source galaxy redshifts is equal to the ratios of their corresponding
angular-diameter distances. This is the so-called shear-ratio test (SRT) and it
is valid when effects induced by the intervening large-scale structure (LSS)
can be neglected. The dominant LSS effect is magnification bias which, on the
one hand, induces an additional shear, and on the other hand, causes a
magnification of the lens population. Our objective is to quantify the
magnification bias for the SRT and show an easy-to-apply mitigation strategy
that does not rely on additional observations. We use ray-tracing data through
the Millennium simulation to measure the influence of magnification on the SRT
and test our mitigation strategy. Using the SRT as a null-test we find
deviations from zero up to for a flux-limited sample of lens galaxies,
which is a strong function of lens redshift and the lens-source line-of-sight
separation. Using our mitigation strategy we can improve the null-test by a
factor of .Comment: 9 pages, 7 figure
Thermodynamic laws in isolated systems
The recent experimental realization of exotic matter states in isolated
quantum systems and the ensuing controversy about the existence of negative
absolute temperatures demand a careful analysis of the conceptual foundations
underlying microcanonical thermostatistics. Here, we provide a detailed
comparison of the most commonly considered microcanonical entropy definitions,
focussing specifically on whether they satisfy or violate the zeroth, first and
second law of thermodynamics. Our analysis shows that, for a broad class of
systems that includes all standard classical Hamiltonian systems, only the
Gibbs volume entropy fulfills all three laws simultaneously. To avoid
ambiguities, the discussion is restricted to exact results and analytically
tractable examples.Comment: footnotes 19, 26 and outlook section adde
Bayesian weak lensing tomography: Reconstructing the 3D large-scale distribution of matter with a lognormal prior
We present a Bayesian reconstruction algorithm that infers the
three-dimensional large-scale matter distribution from the weak gravitational
lensing effects measured in the image shapes of galaxies. The algorithm is
designed to also work with non-Gaussian posterior distributions which arise,
for example, from a non-Gaussian prior distribution. In this work, we use a
lognormal prior and compare the reconstruction results to a Gaussian prior in a
suite of increasingly realistic tests on mock data. We find that in cases of
high noise levels (i.e. for low source galaxy densities and/or high shape
measurement uncertainties), both normal and lognormal priors lead to
reconstructions of comparable quality, but with the lognormal reconstruction
being prone to mass-sheet degeneracy. In the low-noise regime and on small
scales, the lognormal model produces better reconstructions than the normal
model: The lognormal model 1) enforces non-negative densities, while negative
densities are present when a normal prior is employed, 2) better traces the
extremal values and the skewness of the true underlying distribution, and 3)
yields a higher pixel-wise correlation between the reconstruction and the true
density.Comment: 23 pages, 12 figures; updated to match version accepted for
publication in PR
Noiseless Gravitational Lensing Simulations
The microphysical properties of the DM particle can, in principle, be
constrained by the properties and abundance of substructures in DM halos, as
measured through strong gravitational lensing. Unfortunately, there is a lack
of accurate theoretical predictions for the lensing signal of substructures,
mainly because of the discreteness noise inherent to N-body simulations. Here
we present Recursive-TCM, a method that is able to provide lensing predictions
with an arbitrarily low discreteness noise, without any free parameters or
smoothing scale. This solution is based on a novel way of interpreting the
results of N-body simulations, where particles simply trace the evolution and
distortion of Lagrangian phase-space volume elements. We discuss the advantages
of this method over the widely used cloud-in-cells and adaptive-kernel
smoothing density estimators. Applying the new method to a cluster-sized DM
halo simulated in warm and cold DM scenarios, we show how the expected
differences in their substructure population translate into differences in the
convergence and magnification maps. We anticipate that our method will provide
the high-precision theoretical predictions required to interpret and fully
exploit strong gravitational lensing observations.Comment: 13 pages, 13 figures. Updated fig 12, references adde
The cosmological information of shear peaks: beyond the abundance
We study the cosmological information of weak lensing (WL) peaks, focusing on
two other statistics besides their abundance: the stacked tangential-shear
profiles and the peak-peak correlation function. We use a large ensemble of
simulated WL maps with survey specifications relevant to future missions like
Euclid and LSST, to explore the three peak probes. We find that the correlation
function of peaks with high signal-to-noise (S/N) measured from fields of size
144 sq. deg. has a maximum of ~0.3 at an angular scale ~10 arcmin. For peaks
with smaller S/N, the amplitude of the correlation function decreases, and its
maximum occurs on smaller angular scales. We compare the peak observables
measured with and without shape noise and find that for S/N~3 only ~5% of the
peaks are due to large-scale structures, the rest being generated by shape
noise. The covariance matrix of the probes is examined: the correlation
function is only weakly covariant on scales < 30 arcmin, and slightly more on
larger scales; the shear profiles are very correlated for theta > 2 arcmin,
with a correlation coefficient as high as 0.7. Using the Fisher-matrix
formalism, we compute the cosmological constraints for {Om_m, sig_8, w, n_s}
considering each probe separately, as well as in combination. We find that the
correlation function of peaks and shear profiles yield marginalized errors
which are larger by a factor of 2-4 for {Om_m, sig_8} than the errors yielded
by the peak abundance alone, while the errors for {w, n_s} are similar. By
combining the three probes, the marginalized constraints are tightened by a
factor of ~2 compared to the peak abundance alone, the least contributor to the
error reduction being the correlation function. This work therefore recommends
that future WL surveys use shear peaks beyond their abundance in order to
constrain the cosmological model.Comment: 15 pages, 10 figures, submitted to MNRA
Magneto-elastic coupling within the Stoner model
Based on the Stoner Model for a single band in the mean field approximation (MFA),
aspects of the interaction between lattice deformation and magnetisation of itinerant
electron systems are studied.
The derivation of the Hartree–Fock–Stoner (HFS) Hamiltonian is reviewed for a single
band starting from a general Hamiltonian describing band electrons. The finite-temperature
properties of the model, including the various magnetic states and the ferromagnetic-to-paramagnetic phase transition, are briefly discussed within MFA.
The HFS Hamiltonian is applied to a single band with a rectangular density of states
(DOS). The finite temperature magnetic properties of the system are studied using MFA.
The model is extended to incorporate the interaction of lattice and magnetic degrees of
freedom by introducing a dependence of the bandwidth on the lattice parameter. The
effects of local variations of the lattice parameter are studied by introducing a local
bandwidth and treating the variations as fluctuations. The results are compared to
experimental magnetisation measurements of Invar alloys. Furthermore, magnetostriction
and magnetic contributions to the thermal expansion are discussed within the model.
Finally, effects of particle exchanges are considered within the model
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