1,584 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
On Gravitational Waves in Spacetimes with a Nonvanishing Cosmological Constant
We study the effect of a cosmological constant on the propagation
and detection of gravitational waves. To this purpose we investigate the
linearised Einstein's equations with terms up to linear order in in a
de Sitter and an anti-de Sitter background spacetime. In this framework the
cosmological term does not induce changes in the polarization states of the
waves, whereas the amplitude gets modified with terms depending on .
Moreover, if a source emits a periodic waveform, its periodicity as measured by
a distant observer gets modified. These effects are, however, extremely tiny
and thus well below the detectability by some twenty orders of magnitude within
present gravitational wave detectors such as LIGO or future planned ones such
as LISA.Comment: 8 pages, 4 figures, accepted for publication in Physical Review
Evaluation of diffusion MRI based feature sets for the classification of primary motor and somatosensory cortical areas
In the following work several diffusion based feature vectors (DTI, NODDI, spherical harmonic (SH) invariants and fourth order tensor invariants (T4)) are compared in order to validate their usability in grey matter investigations. It was found that using multi-shell data and non-biophysical models such as SH and T4 achieves the highest classification accuracy between the primary motor and somatosensory cortical areas, and thus is likely to characterise grey matter tissues domains more effectively
Combining HARDI datasets with more than one b-value improves diffusion MRI-based cortical parcellation
Examination of draining vein contributions in GE- and SE-EPI BOLD across cortical depth at 7T
Effects of Interplanetary Dust on the LISA drag-free Constellation
The analysis of non-radiative sources of static or time-dependent
gravitational fields in the Solar System is crucial to accurately estimate the
free-fall orbits of the LISA space mission. In particular, we take into account
the gravitational effects of Interplanetary Dust (ID) on the spacecraft
trajectories. The perturbing gravitational field has been calculated for some
ID density distributions that fit the observed zodiacal light. Then we
integrated the Gauss planetary equations to get the deviations from the LISA
keplerian orbits around the Sun. This analysis can be eventually extended to
Local Dark Matter (LDM), as gravitational fields are expected to be similar for
ID and LDM distributions. Under some strong assumptions on the displacement
noise at very low frequency, the Doppler data collected during the whole LISA
mission could provide upper limits on ID and LDM densities.Comment: 11 pages, 6 figures, to be published on the special issue of
"Celestial Mechanics and Dynamical Astronomy" on the CELMEC V conferenc
Solar and stellar system tests of the cosmological constant
Some tests of gravity theories - periastron shift, geodetic precession,
change in mean motion and gravitational redshift - are applied in solar and
stellar systems to constrain the cosmological constant. We thus consider a
length scale range from 10^8 to 10^{15} km. Best bounds from the solar system
come from perihelion advance and change in mean motion of Earth and Mars,
Lambda < 10^{-36} km^{-2}. Such a limit falls very short to estimates from
observational cosmology analyses but a future experiment performing radio
ranging observations of outer planets could improve it by four orders of
magnitude. Beyond the solar system, together with future measurements of
periastron advance in wide binary pulsars, gravitational redshift of white
dwarfs can provide bounds competitive with Mars data.Comment: 4 pages; this is a preprint of an article accepted for publication in
Physical Review
Gravitational lensing in metric theories of gravity
Gravitational lensing in metric theories of gravity is discussed. I introduce
a generalized approximate metric element, inclusive of both post-post-Newtonian
(ppN) contributions and gravito-magnetic field. Following Fermat's principle
and standard hyphoteses, I derive the time delay function and deflection angle
caused by an isolated mass distribution. Several astrophysical systems are
considered. In most of the cases, the gravito-magnetic correction offers the
best perspectives for an observational detection. Actual measurements
distinguish only marginally different metric theories one from another.Comment: 15 pages; to appear in Phys. Rev.
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