8,823 research outputs found
A Free-Form Lensing Grid Solution for A1689 with New Mutiple Images
Hubble Space Telescope imaging of the galaxy cluster Abell 1689 has revealed
an exceptional number of strongly lensed multiply-imaged galaxies, including
high-redshift candidates. Previous studies have used this data to obtain the
most detailed dark matter reconstructions of any galaxy cluster to date,
resolving substructures ~25 kpc across. We examine Abell 1689 (hereafter,
A1689) non-parametrically, combining strongly lensed images and weak
distortions from wider field Subaru imaging, and we incorporate member galaxies
to improve the lens solution. Strongly lensed galaxies are often locally
affected by member galaxies, however, these perturbations cannot be recovered
in grid based reconstructions because the lensing information is too sparse to
resolve member galaxies. By adding luminosity-scaled member galaxy deflections
to our smooth grid we can derive meaningful solutions with sufficient accuracy
to permit the identification of our own strongly lensed images, so our model
becomes self consistent. We identify 11 new multiply lensed system candidates
and clarify previously ambiguous cases, in the deepest optical and NIR data to
date from Hubble and Subaru. Our improved spatial resolution brings up new
features not seen when the weak and strong lensing effects are used separately,
including clumps and filamentary dark matter around the main halo. Our
treatment means we can obtain an objective mass ratio between the cluster and
galaxy components, for examining the extent of tidal stripping of the luminous
member galaxies. We find a typical mass-to-light ratios of M/L_B = 21 inside
the r<1 arcminute region that drops to M/L_B = 17 inside the r<40 arcsecond
region. Our model independence means we can objectively evaluate the
competitiveness of stacking cluster lenses for defining the geometric
lensing-distance-redshift relation in a model independent way.Comment: 23 pages with 25 figures Replced with MNRAS submitted version. Some
figures have been corrected and minor text edit
Effects of Gamma Ray Bursts in Earth Biosphere
We continue former work on the modeling of potential effects of Gamma Ray
Bursts on Phanerozoic Earth. We focus on global biospheric effects of ozone
depletion and show a first modeling of the spectral reduction of light by NO2
formed in the stratosphere. We also illustrate the current complexities
involved in the prediction of how terrestrial ecosystems would respond to this
kind of burst. We conclude that more biological field and laboratory data are
needed to reach even moderate accuracy in this modelingComment: Accepted for publication in Astrophysics & Space Scienc
Measuring hadron properties at finite temperature
We estimate the numbers and mass spectra of observed lepton and kaon pairs
produced from meson decays in the central rapidity region of an Au+Au
collision at lab energy 11.6 GeV/nucleon. The following effects are considered:
possible mass shifts, thermal broadening due to collisions with hadronic
resonances, and superheating of the resonance gas. Changes in the dilepton mass
spectrum may be seen, but changes in the dikaon spectrum are too small to be
detectable.Comment: 9 pages (revtex), 3 figures (uuencoded postscript
Performance analysis of energy detection over hyper-Rayleigh fading channels
This study investigates the performance of energy detection (ED)-based spectrum sensing over two-wave with diffused power (TWDP) fading channels, which have been found to provide accurate characterisation for a variety of fading conditions. A closed-form expression for the average detection probability of ED-based spectrum sensing over TWDP fading channels is derived. This expression is then used to describe the behaviour of ED-based spectrum sensing for a variety of channels that include Rayleigh, Rician and hyper-Rayleigh fading models. Such fading scenarios present a reliable behavioural model of machine-to-machine wireless nodes operating in confined structures such as in-vehicular environments
Discovery of a Ringlike Dark Matter Structure in the Core of the Galaxy Cluster Cl 0024+17
We present a comprehensive mass reconstruction of the rich galaxy cluster Cl
0024+17 at z~0.4 from ACS data, unifying both strong- and weak-lensing
constraints. The weak-lensing signal from a dense distribution of background
galaxies (~120 per square arcmin) across the cluster enables the derivation of
a high-resolution parameter-free mass map. The strongly-lensed objects tightly
constrain the mass structure of the cluster inner region on an absolute scale,
breaking the mass-sheet degeneracy. The mass reconstruction of Cl 0024+17
obtained in such a way is remarkable. It reveals a ringlike dark matter
substructure at r~75" surrounding a soft, dense core at r~50". We interpret
this peculiar sub-structure as the result of a high-speed line-of-sight
collision of two massive clusters 1-2 Gyr ago. Such an event is also indicated
by the cluster velocity distribution. Our numerical simulation with purely
collisionless particles demonstrates that such density ripples can arise by
radially expanding, decelerating particles that originally comprised the
pre-collision cores. Cl 0024+17 can be likened to the bullet cluster 1E0657-56,
but viewed the collision axis at a much later epoch. In addition, we
show that the long-standing mass discrepancy for Cl 0024+17 between X-ray and
lensing can be resolved by treating the cluster X-ray emission as coming from a
superposition of two X-ray systems. The cluster's unusual X-ray surface
brightness profile that requires a two isothermal sphere description supports
this hypothesis.Comment: To appear in the June 1 issue of The Astrophysical Journa
The Sextet Arcs: a Strongly Lensed Lyman Break Galaxy in the ACS Spectroscopic Galaxy Survey towards Abell 1689
We present results of the HST Advanced Camera for Surveys spectroscopic
ground-based redshift survey in the field of A1689. We measure 98 redshifts,
increasing the number of spectroscopically confirmed objects by sixfold. We
present two spectra from this catalog of the Sextet Arcs, images which arise
from a strongly-lensed Lyman Break Galaxy (LBG) at a redshift of z=3.038.
Gravitational lensing by the cluster magnifies its flux by a factor of ~16 and
produces six separate images with a total r-band magnitude of r_625=21.7. The
two spectra, each of which represents emission from different regions of the
LBG, show H I and interstellar metal absorption lines at the systemic redshift.
Significant variations are seen in Ly-alpha profile across a single galaxy,
ranging from strong absorption to a combination of emission plus absorption. A
spectrum of a third image close to the brightest arc shows Ly-alpha emission at
the same redshift as the LBG, arising from either another spatially distinct
region of the galaxy, or from a companion galaxy close to the LBG. Taken as a
group, the Ly-alpha equivalent width in these three spectra decreases with
increasing equivalent width of the strongest interstellar absorption lines. We
discuss how these variations can be used to understand the physical conditions
in the LBG. Intrinsically, this LBG is faint, ~0.1L*, and forming stars at a
modest rate, ~4 solar masses per year. We also detect absorption line systems
toward the Sextet Arcs at z=2.873 and z=2.534. The latter system is seen across
two of our spectra.Comment: Accepted for publication in Ap
Photo-z optimization for measurements of the BAO radial direction
Baryon Acoustic Oscillations (BAO) in the radial direction offer a method to
directly measure the Universe expansion history, and to set limits to space
curvature when combined to the angular BAO signal. In addition to spectroscopic
surveys, radial BAO might be measured from accurate enough photometric
redshifts obtained with narrow-band filters. We explore the requirements for a
photometric survey using Luminous Red Galaxies (LRG) to competitively measure
the radial BAO signal and discuss the possible systematic errors of this
approach. If LRG were a highly homogeneous population, we show that the photo-z
accuracy would not substantially improve by increasing the number of filters
beyond , except for a small fraction of the sources detected at high
signal-to-noise, and broad-band filters would suffice to achieve the target
for measuring radial BAO. Using the LRG spectra
obtained from SDSS, we find that the spectral variability of LRG substantially
worsens the achievable photometric redshift errors, and that the optimal system
consists of 30 filters of width . A
is generally necessary at the filters on the red side of the
break to reach the target photometric accuracy. We estimate that a
5-year survey in a dedicated telescope with etendue in excess of 60 would be necessary to obtain a high enough density of galaxies to
measure radial BAO with sufficiently low shot noise up to . We
conclude that spectroscopic surveys have a superior performance than
photometric ones for measuring BAO in the radial direction.Comment: Replaced with minor editorial comments and one extra figure. Results
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