2,067 research outputs found
Discovery of a Galaxy Cluster via Weak Lensing
We report the discovery of a cluster of galaxies via its weak gravitational
lensing effect on background galaxies, the first spectroscopically confirmed
cluster to be discovered through its gravitational effects rather than by its
electromagnetic radiation. This fundamentally different selection mechanism
promises to yield mass-selected, rather than baryon or photon-selected, samples
of these important cosmological probes. We have confirmed this cluster with
spectroscopic redshifts of fifteen members at z=0.276, with a velocity
dispersion of 615 km/s. We use the tangential shear as a function of source
photometric redshift to estimate the lens redshift independently and find z_l =
0.30 +- 0.08. The good agreement with the spectroscopy indicates that the
redshift evolution of the mass function may be measurable from the imaging data
alone in shear-selected surveys.Comment: revised version with minor changes, to appear in ApJ
A Comparison of Cosmological Hydrodynamic Codes
We present a detailed comparison of the simulation results of various
cosmological hydrodynamic codes. Starting with identical initial conditions
based on the Cold Dark Matter scenario for the growth of structure, we
integrate from redshift to to determine the physical state within
a representative volume of size where . Five
independent codes are compared: three of them Eulerian mesh based and two
variants of the Smooth Particle Hydrodynamics "SPH" Lagrangian approach. The
Eulerian codes were run at cells,
the SPH codes at and particles. Results were then rebinned
to a grid with the expectation that the rebinned data should converge,
by all techniques, to a common and correct result as . We
find that global averages of various physical quantities do, as expected, tend
to converge in the rebinned model, but that uncertainties in even primitive
quantities such as , persists
at the 3\%-17\% level after completion of very large simulations. The two SPH
codes and the two shock capturing Eulerian codes achieve comparable and
satisfactory accuracy for comparable computer time in their treatment of the
high density, high temperature regions as measured in the rebinned data; the
variance among the five codes (at highest resolution) for the mean temperature
(as weighted by ) is only 4.5\%. Overall the comparison allows us to
better estimate errors, it points to ways of improving this current generation
of hydrodynamic codes and of suiting their use to problems which exploit their
individually best features.Comment: 20p plaintex to appear in The Astrophysical Journal on July 20, 199
Integral closure of rings of integer-valued polynomials on algebras
Let be an integrally closed domain with quotient field . Let be a
torsion-free -algebra that is finitely generated as a -module. For every
in we consider its minimal polynomial , i.e. the
monic polynomial of least degree such that . The ring consists of polynomials in that send elements of back to
under evaluation. If has finite residue rings, we show that the
integral closure of is the ring of polynomials in which
map the roots in an algebraic closure of of all the , ,
into elements that are integral over . The result is obtained by identifying
with a -subalgebra of the matrix algebra for some and then
considering polynomials which map a matrix to a matrix integral over . We
also obtain information about polynomially dense subsets of these rings of
polynomials.Comment: Keywords: Integer-valued polynomial, matrix, triangular matrix,
integral closure, pullback, polynomially dense set. accepted for publication
in the volume "Commutative rings, integer-valued polynomials and polynomial
functions", M. Fontana, S. Frisch and S. Glaz (editors), Springer 201
Getting the Measure of the Flatness Problem
The problem of estimating cosmological parameters such as from noisy
or incomplete data is an example of an inverse problem and, as such, generally
requires a probablistic approach. We adopt the Bayesian interpretation of
probability for such problems and stress the connection between probability and
information which this approach makes explicit.
This connection is important even when information is ``minimal'' or, in
other words, when we need to argue from a state of maximum ignorance. We use
the transformation group method of Jaynes to assign minimally--informative
prior probability measure for cosmological parameters in the simple example of
a dust Friedman model, showing that the usual statements of the cosmological
flatness problem are based on an inappropriate choice of prior. We further
demonstrate that, in the framework of a classical cosmological model, there is
no flatness problem.Comment: 11 pages, submitted to Classical and Quantum Gravity, Tex source
file, no figur
Accuracy of Mesh Based Cosmological Hydrocodes: Tests and Corrections
We perform a variety of tests to determine the numerical resolution of the
cosmological TVD eulerian code developed by Ryu et al (1993). Tests include
512^3 and 256^3 simulations of a Pk=k^{-1} spectrum to check for
self-similarity and comparison of results with those from higher resolution SPH
and grid-based calculations (Frenk et al 1998). We conclude that in regions
where density gradients are not produced by shocks the code degrades resolution
with a Gaussian smoothing (radius) length of 1.7 cells. At shock caused
gradients (for which the code was designed) the smoothing length is 1.1 cells.
Finally, for \beta model fit clusters, we can approximately correct numerical
resolution by the transformation R^2_{core}\to R^2_{core}-(C\Delta l)^2, where
\Delta l is the cell size and C=1.1-1.7. When we use these corrections on our
previously published computations for the SCDM and \Lambda CDM models we find
luminosity weighted, zero redshift, X-ray cluster core radii of (210\pm 86,
280\pm 67)h^{-1}kpc, respectively, which are marginally consistent with
observed (Jones & Forman 1992) values of 50-200h^{-1}kpc. Using the corrected
core radii, the COBE normalized SCDM model predicts the number of bright
L_x>10^{43}erg/s clusters too high by a factor of \sim 20 and the \Lambda CDM
model is consistent with observations.Comment: ApJ in press (1999
Fluoropyrimidine sensitivity of human MCF-7 breast cancer cells stably transfected with human uridinehosphorylase
The relationship between uridine phosphorylase (UP) expression level in cancer cells and the tumour sensitivity to fluoropyrimidines is unclear. In this study, we found that UP overexpression by gene transfer, and the subsequent efficient metabolic activation of 5-fluorouracil (5-FU) by the ribonucleotide pathway, does not increase the fluoropyrimidine sensitivity of MCF-7 human cancer cells. Š 2001 Cancer Research Campaign http://www.bjcancer.co
Cross-correlation Weak Lensing of SDSS galaxy Clusters II: Cluster Density Profiles and the Mass--Richness Relation
We interpret and model the statistical weak lensing measurements around
130,000 groups and clusters of galaxies in the Sloan Digital Sky Survey
presented by Sheldon et al. 2007 (Paper I). We present non-parametric
inversions of the 2D shear profiles to the mean 3D cluster density and mass
profiles in bins of both optical richness and cluster i-band luminosity. We
correct the inferred 3D profiles for systematic effects, including non-linear
shear and the fact that cluster halos are not all precisely centered on their
brightest galaxies. We also model the measured cluster shear profile as a sum
of contributions from the brightest central galaxy, the cluster dark matter
halo, and neighboring halos. We infer the relations between mean cluster virial
mass and optical richness and luminosity over two orders of magnitude in
cluster mass; the virial mass at fixed richness or luminosity is determined
with a precision of 13% including both statistical and systematic errors. We
also constrain the halo concentration parameter and halo bias as a function of
cluster mass; both are in good agreement with predictions of LCDM models. The
methods employed here will be applicable to deeper, wide-area optical surveys
that aim to constrain the nature of the dark energy, such as the Dark Energy
Survey, the Large Synoptic Survey Telescope and space-based surveys
Measuring Cluster Peculiar Velocities and Temperatures at cm and mm Wavelengths
We present a detailed investigation of issues related to the measurement of
peculiar velocities and temperatures using Sunyaev-Zel'dovich (SZ) effects. We
estimate the accuracy to which peculiar velocities and gas temperatures of
distant galaxy clusters could be measured. With uK sensitivity on arcminute
scales at several frequencies it will be possible to measure peculiar
velocities to an accuracy of about 130 km/s and gas temperatures to better than
1 keV. The limiting factor for the accuracy of the measured peculiar velocity
is the presence of bulk motions within the galaxy cluster, even for apparently
relaxed clusters. The accuracy of the temperature is mainly limited by noise.
These results are independent of redshift. Such constraints can best be
achieved with only three frequencies: one in the Rayleigh-Jeans region (<40
GHz), one near 150 GHz, and the third at 300 GHz or higher. Measurements at the
null of the thermal SZ effect are of marginal utility, other than as a
foreground/background monitor.Comment: 6 pages, 6 figures, submitted to Ap
Detection of a redshift 3.04 filament
The filamentary structure of the early universe has until now only been seen
in numerical simulations. Despite this lack of direct observational evidence,
the prediction of early filamentary structure formation in a Cold Dark Matter
dominated universe has become a paradigm for our understanding of galaxy
assembly at high redshifts. Clearly observational confirmation is required.
Lyman Break galaxies are too rare to be used as tracers of filaments and we
argue that to map out filaments in the high z universe, one will need to
identify classes of objects fainter than those currently accessible via the
Lyman Break technique. Objects selected via their Ly-alpha emission, and/or as
DLA absorbers, populate the faintest accessible part of the high redshift
galaxy luminosity function, and as such make up good candidates for objects
which will map out high redshift filaments. Here we present the first direct
detection of a filament (at z=3.04) mapped by those classes of objects. The
observations are the deepest yet to have been done in Ly-alpha imaging at high
redshift, and they reveal a single string of proto-galaxies spanning about 5
Mpc (20 Mpc comoving). Expanding the cosmological test proposed by Alcock &
Paczynski (1979), we outline how observations of this type can be used to
determine Omega_Lambda at z=3.Comment: 5 pages, LaTeX, 3 PostScript figures; Accepted for publication in
A&A-Letter
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