379 research outputs found
Gravitational lens magnification by Abell 1689: Distortion of the background galaxy luminosity function
Gravitational lensing magnifies the luminosity of galaxies behind the lens.
We use this effect to constrain the total mass in the cluster Abell 1689 by
comparing the lensed luminosities of background galaxies with the luminosity
function of an undistorted field. Since galaxies are assumed to be a random
sampling of luminosity space, this method is not limited by clustering noise.
We use photometric redshift information to estimate galaxy distance and
intrinsic luminosity. Knowing the redshift distribution of the background
population allows us to lift the mass/background degeneracy common to lensing
analysis. In this paper we use 9 filters observed over 12 hours with the Calar
Alto 3.5m telescope to determine the redshifts of 1000 galaxies in the field of
Abell 1689. Using a complete sample of 151 background galaxies we measure the
cluster mass profile. We find that the total projected mass interior to
0.25h^(-1)Mpc is (0.48 +/- 0.16) * 10^(15)h^(-1) solar masses, where our error
budget includes uncertainties from the photometric redshift determination, the
uncertainty in the off-set calibration and finite sampling. This result is in
good agreement with that found by number count and shear-based methods and
provides a new and independent method to determine cluster masses.Comment: 13 pages, 10 figures. Submitted to MNRAS (10/99); Replacement with 1
page extra text inc. new section, accepted by MNRA
Theory Challenges of the Accelerating Universe
The accelerating expansion of the universe presents an exciting, fundamental
challenge to the standard models of particle physics and cosmology. I highlight
some of the outstanding challenges in both developing theoretical models and
interpreting without bias the observational results from precision cosmology
experiments in the next decade that will return data to help reveal the nature
of the new physics. Examples given focus on distinguishing a new component of
energy from a new law of gravity, and the effect of early dark energy on baryon
acoustic oscillations.Comment: 10 pages, 4 figures; minor changes to match J. Phys. A versio
Probing the distribution of dark matter in the Abell 901/902 supercluster with weak lensing
We present a weak shear analysis of the Abell 901/902 supercluster, composed
of three rich clusters at z=0.16. Using a deep R-band image from the 0.5 x 0.5
degree MPG/ESO Wide Field Imager together with supplementary B-band
observations, we build up a comprehensive picture of the light and mass
distributions in this region. We find that, on average, the light from the
early-type galaxies traces the dark matter fairly well, although one cluster is
a notable exception to this rule. The clusters themselves exhibit a range of
mass-to-light (M/L) ratios, X-ray properties, and galaxy populations. We
attempt to model the relation between the total mass and the light from the
early-type galaxies with a simple scale-independent linear biasing model. We
find M/L_B=130h for the early type galaxies with zero stochasticity, which, if
taken at face value, would imply Omega_m < 0.1. However, this linear relation
breaks down on small scales and on scales equivalent to the average cluster
separation (approximately 1 Mpc), demonstrating that a single M/L ratio is not
adequate to fully describe the mass-light relation in the supercluster. Rather,
the scatter in M/L ratios observed for the clusters supports a model
incorporating non-linear biasing or stochastic processes. Finally, there is a
clear detection of filamentary structure connecting two of the clusters, seen
in both the galaxy and dark matter distributions, and we discuss the effects of
cluster-cluster and cluster-filament interactions as a means to reconcile the
disparate descriptions of the supercluster.Comment: 23 pages, 19 figures. ApJ, accepte
An optical study of the GRB 970111 field beginning 19 hours after the Gamma-Ray Burst
We present the results of the monitoring of the GRB 970111 field that started
19 hours after the event. This observation represents the fastest ground-based
follow-up performed for GRB 970111 in all wavelengths. As soon as the detection
of the possible GRB 970111 X-ray afterglow was reported by Feroci et al. (1998)
we reanalyzed the optical data collected for the GRB 970111 field. Although we
detect small magnitude variability in some objects, no convincing optical
counterpart is found inside the WFC error box. Any change in brightness 19
hours after the GRB is less than 0.2 mag for objects with B < 21 and R < 20.8.
The bluest object found in the field is coincident with 1SAXJ1528.8+1937.
Spectroscopic observations revealed that this object is a Seyfert-1 galaxy with
redshift z=0.657, which we propose as the optical counterpart of the X-ray
source.
Further observations allowed to perform multicolour photometry for objects in
the GRB 970111 error box. The colour-colour diagrams do not show any object
with unusual colours. We applied a photometric classification method to the
objects inside the GRB error box, that can distinguish stars from galaxies and
estimate redshifts. We were able to estimate photometric redshifts in the range
0.2 < z < 1.4 for several galaxies in this field and we did not find any
conspicuous unusual object.
We note that GRB 970111 and GRB 980329 could belong to the same class of
GRBs, which may be related to nearby sources (z ~1) in which high intrinsic
absorption leads to faint optical afterglows.Comment: 10 pages with 11 encapsulated PostScript figures included. Uses
Astronomy & Astrophysics LaTeX macros. Accepted for publication in Astronomy
& Astrophysic
Dynamical Shakeup of Planetary Systems II. N-body simulations of Solar System terrestrial planet formation induced by secular resonance sweeping
We revisit the "dynamical shakeup" model of Solar System terrestrial planet
formation, wherein the whole process is driven by the sweeping of Jupiter's
secular resonance as the gas disk is removed. Using a large number of 0.5
Gyr-long N-body simulations, we investigate the different outcomes produced by
such a scenario. We confirm that in contrast to existing models, secular
resonance sweeping combined with tidal damping by the disk gas can reproduce
the low eccentricities and inclinations, and high radial mass concentration, of
the Solar System terrestrial planets. At the same time, this also drives the
final assemblage of the planets on a timescale of several tens of millions of
years, an order of magnitude faster than inferred from previous numerical
simulations which neglected these effects, but possibly in better agreement
with timescales inferred from cosmochemical data. In addition, we find that
significant delivery of water-rich material from the outer Asteroid Belt is a
natural byproduct.Comment: To appear in Ap
The formation of Uranus and Neptune among Jupiter and Saturn
The outer giant planets, Uranus and Neptune, pose a challenge to theories of
planet formation. They exist in a region of the Solar System where long
dynamical timescales and a low primordial density of material would have
conspired to make the formation of such large bodies ( 15 and 17 times as
massive as the Earth, respectively) very difficult. Previously, we proposed a
model which addresses this problem: Instead of forming in the trans-Saturnian
region, Uranus and Neptune underwent most of their growth among proto-Jupiter
and -Saturn, were scattered outward when Jupiter acquired its massive gas
envelope, and subsequently evolved toward their present orbits. We present the
results of additional numerical simulations, which further demonstrate that the
model readily produces analogues to our Solar System for a wide range of
initial conditions. We also find that this mechanism may partly account for the
high orbital inclinations observed in the Kuiper belt.Comment: Submitted to AJ; 38 pages, 16 figure
Capillary condensation in disordered porous materials: hysteresis versus equilibrium behavior
We study the interplay between hysteresis and equilibrium behavior in
capillary condensation of fluids in mesoporous disordered materials via a
mean-field density functional theory of a disordered lattice-gas model. The
approach reproduces all major features observed experimentally. We show that
the simple van der Waals picture of metastability fails due to the appearance
of a complex free-energy landscape with a large number of metastable states. In
particular, hysteresis can occur both with and without an underlying
equilibrium transition, thermodynamic consistency is not satisfied along the
hysteresis loop, and out-of-equilibrium phase transitions are possible.Comment: 4 pages, 4 figure
Simple Model of Capillary Condensation in porous media
We employ a simple model to describe the phase behavior of 4He and Ar in a
hypothetical porous material consisting of a regular array of infinitely long,
solid, parallel cylinders. We find that high porosity geometries exhibit two
transitions: from vapor to film and from film to capillary condensed liquid. At
low porosity, the film is replaced by a ``necking'' configuration, and for a
range of intermediate porosity there are three transitions: from vapor to film,
from film to necking and from necking to a capillary condensed phase.Comment: 14 pages, 7 figure
The Calar Alto Deep Imaging Survey: K-band Galaxy Number Counts
We present K-band number counts for the faint galaxies in the Calar Alto Deep
Imaging Survey (CADIS). We covered 4 CADIS fields, a total area of 0.2deg^2, in
the broad band filters B, R and K. We detect about 4000 galaxies in the K-band
images, with a completeness limit of K=19.75mag, and derive the K-band galaxy
number counts in the range of 14.25 < K < 19.75mag. This is the largest medium
deep K-band survey to date in this magnitude range. The B- and R-band number
counts are also derived, down to completeness limits of B=24.75mag and
R=23.25mag. The K-selected galaxies in this magnitude range are of particular
interest, since some medium deep near-infrared surveys have identified breaks
of both the slope of the K-band number counts and the mean B-K color at
K=17\sim18mag. There is, however, a significant disagreement in the K-band
number counts among the existing surveys. Our large near-infrared selected
galaxy sample allows us to establish the presence of a clear break in the slope
at K=17.0mag from dlogN/dm = 0.64 at brighter magnitudes to dlogN/dm = 0.36 at
the fainter end. We construct no-evolution and passive evolution models, and
find that the passive evolution model can simultaneously fit the B-, R- and
K-band number counts well. The B-K colors show a clear trend to bluer colors
for K > 18mag. We also find that most of the K=18-20mag galaxies have a B-K
color bluer than the prediction of a no-evolution model for an L_* Sbc galaxy,
implying either significant evolution, even for massive galaxies, or the
existence of an extra population of small galaxies.Comment: Accepted for A&A, 10 pages, 7 figure
Lattice model of gas condensation within nanopores
We explore the thermodynamic behavior of gases adsorbed within a nanopore.
The theoretical description employs a simple lattice gas model, with two
species of site, expected to describe various regimes of adsorption and
condensation behavior. The model includes four hypothetical phases: a
cylindrical shell phase (S), in which the sites close to the cylindrical wall
are occupied, an axial phase (A), in which sites along the cylinder's axis are
occupied, a full phase (F), in which all sites are occupied, and an empty phase
(E). We obtain exact results at T=0 for the phase behavior, which is a function
of the interactions present in any specific problem. We obtain the
corresponding results at finite T from mean field theory. Finally, we examine
the model's predicted phase behavior of some real gases adsorbed in nanopores
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