11,343 research outputs found
Weak Lensing Probes of Modified Gravity
We study the effect of modifications to General Relativity on large scale
weak lensing observables. In particular, we consider three modified gravity
scenarios: f(R) gravity, the DGP model, and TeVeS theory. Weak lensing is
sensitive to the growth of structure and the relation between matter and
gravitational potentials, both of which will in general be affected by modified
gravity. Restricting ourselves to linear scales, we compare the predictions for
galaxy-shear and shear-shear correlations of each modified gravity cosmology to
those of an effective Dark Energy cosmology with the same expansion history. In
this way, the effects of modified gravity on the growth of perturbations are
separated from the expansion history. We also propose a test which isolates the
matter-potential relation from the growth factor and matter power spectrum. For
all three modified gravity models, the predictions for galaxy and shear
correlations will be discernible from those of Dark Energy with very high
significance in future weak lensing surveys. Furthermore, each model predicts a
measurably distinct scale dependence and redshift evolution of galaxy and shear
correlations, which can be traced back to the physical foundations of each
model. We show that the signal-to-noise for detecting signatures of modified
gravity is much higher for weak lensing observables as compared to the ISW
effect, measured via the galaxy-CMB cross-correlation.Comment: 16 pages, 8 figures; accepted for publication in Phys. Rev. D; v2:
references added; v3: clarifications and additions to the text in response to
refere
Testing the simplifying assumption in high-dimensional vine copulas
Testing the simplifying assumption in high-dimensional vine copulas is a
difficult task. Tests must be based on estimated observations and amount to
checking constraints on high-dimensional distributions. So far, corresponding
tests have been limited to single conditional copulas with a low-dimensional
set of conditioning variables. We propose a novel testing procedure that is
computationally feasible for high-dimensional data sets and that exhibits a
power that decreases only slightly with the dimension. By discretizing the
support of the conditioning variables and incorporating a penalty in the test
statistic, we mitigate the curse of dimensions by looking for the possibly
strongest deviation from the simplifying assumption. The use of a decision tree
renders the test computationally feasible for large dimensions. We derive the
asymptotic distribution of the test and analyze its finite sample performance
in an extensive simulation study. The utility of the test is demonstrated by
its application to six data sets with up to 49 dimensions
Chandra constraints on the thermal conduction in the intracluster plasma of A2142
In this Letter, we use the recent Chandra observation of A2142 reported by
Markevitch et al. to put constraints on thermal conduction in the intracluster
plasma. We show that the observed sharp temperature gradient requires that
classical conductivity has to be reduced at least by a factor of between 250
and 2500. The result provides a direct constraint on an important physical
process relevant to the gas in the cores of clusters of galaxies.Comment: 3 pages. To appear in MNRA
Band-Structure Effects in the Spin Relaxation of Conduction Electrons
Spin relaxation of conduction electrons in metals is significantly influenced
by the Fermi surface topology. Electrons near Brillouin zone boundaries,
special symmetry points, or accidental degeneracy lines have spin flip rates
much higher than an average electron. A realistic calculation and analytical
estimates show that these regions dominate the spin relaxation, explaining why
polyvalent metals have much higher spin relaxation rates (up to three orders of
magnitude) than similar monovalent metals. This suggests that spin relaxation
in metals can be tailored by band-structure modifications like doping,
alloying, reducing the dimensionality, etc.Comment: 10 pages, 2 figures; to appear in the 43rd MMM Conference Proceedings
published in the JA
Chandra observations of Abell 2199
We present results from an analysis of two Chandra observations of the rich,
nearby galaxy cluster Abell 2199. We find evidence (having corrected for
projection effects) for radial gradients in temperature and metallicity in the
X-ray emitting gas: the temperature drops from kT~4.2 keV at R=200 kpc to 1.6
keV within R=5 kpc of the centre. The metallicity rises from ~0.3 solar at
R=200 kpc to ~0.7 solar at R=30 kpc before dropping to 0.3 solar within the
central 5 kpc. We find evidence for structure in the surface brightness
distribution associated with the central radio source 3C338. No evidence is
found for the gas having a large spread in temperature at any particular
location despite the cooling time being short (<10**9yr) within the central ~15
kpc. Heating and mass cooling rates are calculated for various assumptions
about the state of the gas.Comment: 10 pages, 12 figures. Accepted by MNRAS. Minor changes following
referee's comment
Adaptive binning of X-ray galaxy cluster images
We present a simple method for adaptively binning the pixels in an image. The
algorithm groups pixels into bins of size such that the fractional error on the
photon count in a bin is less than or equal to a threshold value, and the size
of the bin is as small as possible. The process is particularly useful for
generating surface brightness and colour maps, with clearly defined error maps,
from images with a large dynamic range of counts, for example X-ray images of
galaxy clusters. We demonstrate the method in application to data from Chandra
ACIS-S and ACIS-I observations of the Perseus cluster of galaxies. We use the
algorithm to create intensity maps, and colour images which show the relative
X-ray intensities in different bands. The colour maps can later be converted,
through spectral models, into maps of physical parameters, such as temperature,
column density, etc. The adaptive binning algorithm is applicable to a wide
range of data, from observations or numerical simulations, and is not limited
to two-dimensional data.Comment: 8 pages, 12 figures, accepted by MNRAS (includes changes suggested by
referee), high resolution version at
http://www-xray.ast.cam.ac.uk/~jss/adbin
The relationship between cooling flows and metallicity measurements for X-ray luminous clusters
We explore the relationship between the metallicity of the intracluster gas
in clusters of galaxies, determined by X-ray spectroscopy, and the presence of
cooling flows. Using ASCA spectra and ROSAT images, we demonstrate a clear
segregation between the metallicities of clusters with and without cooling
flows. On average, cooling-flow clusters have an emission-weighted metallicity
a factor ~ 1.8 times higher than that of non-cooling flow systems. We suggest
this to be due to the presence of metallicity gradients in the cooling flow
clusters, coupled with the sharply peaked X-ray surface brightness profiles of
these systems. Non-cooling flow clusters have much flatter X-ray surface
brightness distributions and are thought to have undergone recent merger events
which may have mixed the central high-metallicity gas with the surrounding less
metal-rich material. We find no evidence for evolution in the emission-weighted
metallicities of clusters within z~0.3.Comment: Submitted to MNRAS letters (December 1997). 6 pages, 2 figures in
MNRAS LaTex style. Minor revision
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