4,189 research outputs found
Self Calibration of Tomographic Weak Lensing for the Physics of Baryons to Constrain Dark Energy
Numerical studies indicate that uncertainties in the treatment of baryonic
physics can affect predictions for shear power spectra at a level that is
significant for forthcoming surveys such as DES, SNAP, and LSST.
Correspondingly, we show that baryonic effects can significantly bias dark
energy parameter measurements. Eliminating such biases by neglecting
information in multipoles beyond several hundred leads to weaker parameter
constraints by a factor of approximately 2 to 3 compared with using information
out to multipoles of several thousand. Fortunately, the same numerical studies
that explore the influence of baryons indicate that they primarily affect power
spectra by altering halo structure through the relation between halo mass and
mean effective halo concentration. We explore the ability of future weak
lensing surveys to constrain both the internal structures of halos and the
properties of the dark energy simultaneously as a first step toward self
calibrating for the physics of baryons. This greatly reduces parameter biases
and no parameter constraint is degraded by more than 40% in the case of LSST or
30% in the cases of SNAP or DES. Modest prior knowledge of the halo
concentration relation greatly improves even these forecasts. Additionally, we
find that these surveys can constrain effective halo concentrations near
m~10^14 Msun/h and z~0.2 to better than 10% with shear power spectra alone.
These results suggest that inferring dark energy parameters with measurements
of shear power spectra can be made robust to baryonic effects and may
simultaneously be competitive with other methods to inform models of galaxy
formation. (Abridged)Comment: 18 pages, 11 figures. Minor changes reflecting referee's comments.
Results and conclusions unchanged. Accepted for publication in Physical
Review
Weighing the Cosmological Energy Contents with Weak Gravitational Lensing
Bernardeau et al. (1997), using perturbation theory, showed that the skewness
of the large-scale lensing-convergence, or projected mass density, could be
used to constrain , the matter content of the universe. On the other
hand, deep weak-lensing field surveys in the near future will likely measure
the convergence on small angular scales (< 10 arcmin.), where the signal will
be dominated by highly nonlinear fluctuations. We develop a method to compute
the small-scale convergence skewness, using a prescription for the highly
nonlinear three-point function developed by Scoccimarro and Frieman (1998).
This method gives predictions that agree well with existing results from
ray-tracing N-body simulations, but is significantly faster, allowing the
exploration of a large number of models. We demonstrate that the small-scale
convergence skewness is insensitive to the shape and normalization of the
primordial (CDM-type) power spectrum, making it dependent almost entirely on
the cosmological energy contents, through their influence on the global
geometrical distances and fluctuation growth rate. Moreover, nonlinear
clustering appears to enhance the differences between predictions of the
convergence skewness for a range of models. Hence, in addition to constraining
, the small-scale convergence skewness from future deep several-
degree-wide surveys can be used to differentiate between curvature dominated
and cosmological constant () dominated models, as well as to constrain
the equation of state of a quintessence component, thereby distinguishing
from quintessence as well. Finally, our method can be easily
generalized to other measures such as aperture mass statistics.Comment: 13 pages, 2 ps figures, submitted to ApJ
Expectations For an Interferometric Sunyaev-Zel'dovich Effect Survey for Galaxy Clusters
Non-targeted surveys for galaxy clusters using the Sunyaev-Zel'dovich effect
(SZE) will yield valuable information on both cosmology and evolution of the
intra-cluster medium (ICM). The redshift distribution of detected clusters will
constrain cosmology, while the properties of the discovered clusters will be
important for studies of the ICM and galaxy formation. Estimating survey yields
requires a detailed model for both cluster properties and the survey strategy.
We address this by making mock observations of galaxy clusters in cosmological
hydrodynamical simulations. The mock observatory consists of an interferometric
array of ten 2.5 m diameter telescopes, operating at a central frequency of 30
GHz with a bandwidth of 8 GHz. We find that clusters with a mass above will be detected at any redshift, with the
exact limit showing a very modest redshift dependence. Using a Press-Schechter
prescription for evolving the number densities of clusters with redshift, we
determine that such a survey should find hundreds of galaxy clusters per year,
many at high redshifts and relatively low mass -- an important regime uniquely
accessible to SZE surveys. Currently favored cosmological models predict
roughly 25 clusters per square degree.Comment: revised to match published versio
Theoretical and experimental studies of the underlying processes and techniques of low pressure measurement Letter report, 1 Jul. 1967 - 31 May 1968
Ionization gage behavior in various gas mixtures and atmospheric composition
Theoretical and experimental studies of the underlying processes and techniques of low pressure measurement Progress report, 1 Jun. 1966 - 31 May 1967
Construction, testing, and refining of vacuum gauges - ionization gauge cross section ratio
Evidence for a constant IMF in early-type galaxies based on their X-ray binary populations
A number of recent studies have proposed that the stellar initial mass
function (IMF) of early type galaxies varies systematically as a function of
galaxy mass, with higher mass galaxies having bottom heavy IMFs. These bottom
heavy IMFs have more low-mass stars relative to the number of high mass stars,
and therefore naturally result in proportionally fewer neutron stars and black
holes. In this paper, we specifically predict the variation in the number of
black holes and neutron stars based on the power-law IMF variation required to
reproduce the observed mass-to-light ratio trends with galaxy mass. We then
test whether such variations are observed by studying the field low-mass X-ray
binary populations (LMXBs) of nearby early-type galaxies. In these binaries, a
neutron star or black hole accretes matter from a low-mass donor star. Their
number is therefore expected to scale with the number of black holes and
neutron stars present in a galaxy. We find that the number of LMXBs per K-band
light is similar among the galaxies in our sample. These data therefore
demonstrate the uniformity of the slope of the IMF from massive stars down to
those now dominating the K-band light, and are consistent with an invariant
IMF. Our results are inconsistent with an IMF which varies from a
Kroupa/Chabrier like IMF for low mass galaxies to a steep power-law IMF (with
slope =2.8) for high mass galaxies. We discuss how these observations
constrain the possible forms of the IMF variations and how future Chandra
observations can enable sharper tests of the IMF.Comment: 12 pages, 5 figures, 2 tables, submitted to Ap
Patient reactions to a web-based cardiovascular risk calculator in type 2 diabetes: a qualitative study in primary care.
Use of risk calculators for specific diseases is increasing, with an underlying assumption that they promote risk reduction as users become better informed and motivated to take preventive action. Empirical data to support this are, however, sparse and contradictory
Closed form solution for a double quantum well using Gr\"obner basis
Analytical expressions for spectrum, eigenfunctions and dipole matrix
elements of a square double quantum well (DQW) are presented for a general case
when the potential in different regions of the DQW has different heights and
effective masses are different. This was achieved by Gr\"obner basis algorithm
which allows to disentangle the resulting coupled polynomials without
explicitly solving the transcendental eigenvalue equation.Comment: 4 figures, Mathematica full calculation noteboo
Statistical Tests for CHDM and \LambdaCDM Cosmologies
We apply several statistical estimators to high-resolution N-body simulations
of two currently viable cosmological models: a mixed dark matter model, having
contributed by two massive neutrinos (C+2\nuDM), and a Cold
Dark Matter model with Cosmological Constant (\LambdaCDM) with
and h=0.7. Our aim is to compare simulated galaxy samples with the
Perseus-Pisces redshift survey (PPS). We consider the n-point correlation
functions (n=2-4), the N-count probability functions P_N, including the void
probability function P_0, and the underdensity probability function U_\epsilon
(where \epsilon fixes the underdensity threshold in percentage of the average).
We find that P_0 (for which PPS and CfA2 data agree) and P_1 distinguish
efficiently between the models, while U_\epsilon is only marginally
discriminatory. On the contrary, the reduced skewness and kurtosis are,
respectively, S_3\simeq 2.2 and S_4\simeq 6-7 in all cases, quite independent
of the scale, in agreement with hierarchical scaling predictions and estimates
based on redshift surveys. Among our results, we emphasize the remarkable
agreement between PPS data and C+2\nuDM in all the tests performed. In
contrast, the above \LambdaCDM model has serious difficulties in reproducing
observational data if galaxies and matter overdensities are related in a simple
way.Comment: 12 pages, 10 figures, LaTeX (aaspp4 macro), in press on ApJ, Vol.
479, April 199
Mapping the 3-D Dark Matter potential with weak shear
We investigate the practical implementation of Taylor's (2002) 3-dimensional
gravitational potential reconstruction method using weak gravitational lensing,
together with the requisite reconstruction of the lensing potential. This
methodology calculates the 3-D gravitational potential given a knowledge of
shear estimates and redshifts for a set of galaxies. We analytically estimate
the noise expected in the reconstructed gravitational field, taking into
account the uncertainties associated with a finite survey, photometric redshift
uncertainty, redshift-space distortions, and multiple scattering events. In
order to implement this approach for future data analysis, we simulate the
lensing distortion fields due to various mass distributions. We create
catalogues of galaxies sampling this distortion in three dimensions, with
realistic spatial distribution and intrinsic ellipticity for both ground-based
and space-based surveys. Using the resulting catalogues of galaxy position and
shear, we demonstrate that it is possible to reconstruct the lensing and
gravitational potentials with our method. For example, we demonstrate that a
typical ground-based shear survey with redshift limit z=1 and photometric
redshifts with error Delta z=0.05 is directly able to measure the 3-D
gravitational potential for mass concentrations >10^14 M_\odot between
0.1<z<0.5, and can statistically measure the potential at much lower mass
limits. The intrinsic ellipticity of objects is found to be a serious source of
noise for the gravitational potential, which can be overcome by Wiener
filtering or examining the potential statistically over many fields. We examine
the use of the 3-D lensing potential to measure mass and position of clusters
in 3-D, and to detect clusters behind clusters.Comment: 21 pages, including 24 figures, submitted to MNRA
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