93,218 research outputs found
An Optical Readout TPC (O-TPC) for Studies in Nuclear Astrophysics With Gamma-Ray Beams at HIgS
We report on the construction, tests, calibrations and commissioning of an
Optical Readout Time Projection Chamber (O-TPC) detector operating with a
CO2(80%) + N2(20%) gas mixture at 100 and 150 Torr. It was designed to measure
the cross sections of several key nuclear reactions involved in stellar
evolution. In particular, a study of the rate of formation of oxygen and carbon
during the process of helium burning will be performed by exposing the chamber
gas to intense nearly mono-energetic gamma-ray beams at the High Intensity
Gamma Source (HIgS) facility. The O-TPC has a sensitive target-drift volume of
30x30x21 cm^3. Ionization electrons drift towards a double parallel grid
avalanche multiplier, yielding charge multiplication and light emission.
Avalanche induced photons from N2 emission are collected, intensified and
recorded with a Charge Coupled Device (CCD) camera, providing two-dimensional
track images. The event's time projection (third coordinate) and the deposited
energy are recorded by photomultipliers and by the TPC charge-signal,
respectively. A dedicated VME-based data acquisition system and associated data
analysis tools were developed to record and analyze these data. The O-TPC has
been tested and calibrated with 3.183 MeV alpha-particles emitted by a 148Gd
source placed within its volume with a measured energy resolution of 3.0%.
Tracks of alpha and 12C particles from the dissociation of 16O and of three
alpha-particles from the dissociation of 12C have been measured during initial
in-beam test experiments performed at the HIgS facility at Duke University. The
full detection system and its performance are described and the results of the
preliminary in-beam test experiments are reported.Comment: Supported by the Richard F. Goodman Yale-Weizmann Exchange Program,
ACWIS, NY, and USDOE grant Numbers: DE-FG02-94ER40870 and DE-FG02-97ER4103
Non-parametric strong lens inversion of Cl~0024+1654: illustrating the monopole degeneracy
The cluster lens Cl 0024+1654 is undoubtedly one of the most beautiful
examples of strong gravitational lensing, providing five large images of a
single source with well-resolved substructure. Using the information contained
in the positions and the shapes of the images, combined with the null space
information, a non-parametric technique is used to infer the strong lensing
mass map of the central region of this cluster. This yields a strong lensing
mass of 1.60x10^14 M_O within a 0.5' radius around the cluster center. This
mass distribution is then used as a case study of the monopole degeneracy,
which may be one of the most important degeneracies in gravitational lensing
studies and which is extremely hard to break. We illustrate the monopole
degeneracy by adding circularly symmetric density distributions with zero total
mass to the original mass map of Cl 0024+1654. These redistribute mass in
certain areas of the mass map without affecting the observed images in any way.
We show that the monopole degeneracy and the mass-sheet degeneracy together lie
at the heart of the discrepancies between different gravitational lens
reconstructions that can be found in the literature for a given object, and
that many images/sources, with an overall high image density in the lens plane,
are required to construct an accurate, high-resolution mass map based on
strong-lensing data.Comment: 9 pages, accepted for publication by MNRA
Triaxial orbit based galaxy models with an application to the (apparent) decoupled core galaxy NGC 4365
We present a flexible and efficient method to construct triaxial dynamical
models of galaxies with a central black hole, using Schwarzschild's orbital
superposition approach. Our method is general and can deal with realistic
luminosity distributions, which project to surface brightness distributions
that may show position angle twists and ellipticity variations. The models are
fit to measurements of the full line-of-sight velocity distribution (wherever
available). We verify that our method is able to reproduce theoretical
predictions of a three-integral triaxial Abel model. In a companion paper (van
de Ven, de Zeeuw & van den Bosch), we demonstrate that the method recovers the
phase-space distribution function. We apply our method to two-dimensional
observations of the E3 galaxy NGC 4365, obtained with the integral-field
spectrograph SAURON, and study its internal structure, showing that the
observed kinematically decoupled core is not physically distinct from the main
body and the inner region is close to oblate axisymmetric.Comment: 21 Pages, 14 (Colour) Figures, Companion paper is arXiv:0712.0309
Accepted to MNRAS. Full resolution version at
http://www.strw.leidenuniv.nl/~bosch/papers/RvdBosch_triaxmethod.pd
2-Dust : a Dust Radiative Transfer Code for an Axisymmetric System
We have developed a general purpose dust radiative transfer code for an
axisymmetric system, 2-Dust, motivated by the recent increasing availability of
high-resolution images of circumstellar dust shells at various wavelengths.
This code solves the equation of radiative transfer following the principle of
long characteristic in a 2-D polar grid while considering a 3-D radiation field
at each grid point. A solution is sought through an iterative scheme in which
self-consistency of the solution is achieved by requiring a global luminosity
constancy throughout the shell. The dust opacities are calculated through Mie
theory from the given size distribution and optical properties of the dust
grains. The main focus of the code is to obtain insights on (1) the global
energetics of dust grains in the shell (2) the 2-D projected morphologies that
are strongly dependent on the mixed effects of the axisymmetric dust
distribution and inclination angle of the shell. Here, test models are
presented with discussion of the results. The code can be supplied with a
user-defined density distribution function, and thus, is applicable to a
variety of dusty astronomical objects possessing the axisymmetric geometry.Comment: To be published in ApJ, April 2003 issue; 13 pages, 4 tables, 17
figures, 5-page appendix (no figures for the main text included in this
preprint). For the complete preprint and code distribution, contact the
author
Non-parametric inversion of gravitational lensing systems with few images using a multi-objective genetic algorithm
Galaxies acting as gravitational lenses are surrounded by, at most, a handful
of images. This apparent paucity of information forces one to make the best
possible use of what information is available to invert the lens system. In
this paper, we explore the use of a genetic algorithm to invert in a
non-parametric way strong lensing systems containing only a small number of
images. Perhaps the most important conclusion of this paper is that it is
possible to infer the mass distribution of such gravitational lens systems
using a non-parametric technique. We show that including information about the
null space (i.e. the region where no images are found) is prerequisite to avoid
the prediction of a large number of spurious images, and to reliably
reconstruct the lens mass density. While the total mass of the lens is usually
constrained within a few percent, the fidelity of the reconstruction of the
lens mass distribution depends on the number and position of the images. The
technique employed to include null space information can be extended in a
straightforward way to add additional constraints, such as weak lensing data or
time delay information.Comment: 9 pages, accepted for publication by MNRA
Dynamics of the Boxy Elliptical Galaxy NGC 1600
We use three--integral models to infer the distribution function (DF) of the
boxy E3-E4 galaxy NGC 1600 from surface brightness and line profile data on the
minor and major axes. We assume axisymmetry and that the mass-to-light ratio is
constant in the central ~1 R_e. Stars in the resulting gravitational potential
move mainly on regular orbits. We use an approximate third integral K from
perturbation theory, and write the DF as a sum of basis functions in the three
integrals E, L_z and K. We then fit the projected moments of these basis
functions to the kinematic observables and deprojected density, using a
non-parametric algorithm.
The deduced dynamical structure is radially anisotropic, with
sigma_theta/sigma_r ~ sigma_phi/sigma_r ~ 0.7 on the major axis. Both on the
minor axis and near the centre the velocity distribution is more isotropic;
thus the model is flattened by equatorial radial orbits. The kinematic data is
fit without need for a central black hole; the central mass determined
previously from ground-based data therefore overestimates the actual black hole
mass. The mass-to-light ratio of the stars is M/L_V = 6 h_50.
The anisotropy structure of NGC 1600 with a radially anisotropic main body
and more nearly isotropic centre is similar to that found recently in NGC 1399,
NGC 2434, NGC 3379 and NGC 6703, suggesting that this pattern may be common
amongst massive elliptical galaxies. We discuss a possible merger origin of NGC
1600 in the light of these results.Comment: 14 pages, 9 figures, re-submitted to Monthly Notice
Intrinsic alignment of simulated galaxies in the cosmic web: implications for weak lensing surveys
The intrinsic alignment of galaxy shapes (by means of their angular momentum)
and their cross-correlation with the surrounding dark matter tidal field are
investigated using the 160 000, z=1.2 synthetic galaxies extracted from the
high-resolution cosmological hydrodynamical simulation Horizon-AGN. One- and
two-point statistics of the spin of the stellar component are measured as a
function of mass and colour. For the low-mass galaxies, this spin is locally
aligned with the tidal field `filamentary' direction while, for the high-mass
galaxies, it is perpendicular to both filaments and walls. The bluest galaxies
of our synthetic catalog are more strongly correlated with the surrounding
tidal field than the reddest galaxies, and this correlation extends up to 10
Mpc/h comoving distance. We also report a correlation of the projected
ellipticities of blue, intermediate mass galaxies on a similar scale at a level
of 10^(-4) which could be a concern for cosmic shear measurements. We do not
report any measurable intrinsic alignments of the reddest galaxies of our
sample. This work is a first step toward the use of very realistic catalog of
synthetic galaxies to evaluate the contamination of weak lensing measurement by
the intrinsic galactic alignments.Comment: 15 pages, accepted for publication in MNRA
Non-parametric strong lens inversion of SDSS J1004+4112
In this article we study the well-known strong lensing system SDSS
J1004+4112. Not only does it host a large-separation lensed quasar with
measured time-delay information, but several other lensed galaxies have been
identified as well. A previously developed strong lens inversion procedure that
is designed to handle a wide variety of constraints, is applied to this lensing
system and compared to results reported in other works. Without the inclusion
of a tentative central image of one of the galaxies as a constraint, we find
that the model recovered by the other constraints indeed predicts an image at
that location. An inversion which includes the central image provides tighter
constraints on the shape of the central part of the mass map. The resulting
model also predicts a central image of a second galaxy where indeed an object
is visible in the available ACS images. We find masses of 2.5x10^13 M_O and
6.1x10^13 M_O within a radius of 60 kpc and 110 kpc respectively, confirming
the results from other authors. The resulting mass map is compatible with an
elliptical generalization of a projected NFW profile, with r_s = 58_{-13}^{+21}
arcsec and c_vir = 3.91 +/- 0.74. The orientation of the elliptical NFW profile
follows closely the orientation of the central cluster galaxy and the overall
distribution of cluster members.Comment: 11 pages, accepted for publication in MNRA
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