164 research outputs found
Mass-sheet degeneracy, power-law models and external convergence: Impact on the determination of the Hubble constant from gravitational lensing
The light travel time differences in strong gravitational lensing systems
allows an independent determination of the Hubble constant. This method has
been successfully applied to several lens systems. The formally most precise
measurements are, however, in tension with the recent determination of
from the Planck satellite for a spatially flat six-parameters
cosmology. We reconsider the uncertainties of the method, concerning the mass
profile of the lens galaxies, and show that the formal precision relies on the
assumption that the mass profile is a perfect power law. Simple analytical
arguments and numerical experiments reveal that mass-sheet like transformations
yield significant freedom in choosing the mass profile, even when exquisite
Einstein rings are observed. Furthermore, the characterization of the
environment of the lens does not break that degeneracy which is not physically
linked to extrinsic convergence. We present an illustrative example where the
multiple imaging properties of a composite (baryons + dark matter) lens can be
extremely well reproduced by a power-law model having the same velocity
dispersion, but with predictions for the Hubble constant that deviate by . Hence we conclude that the impact of degeneracies between parametrized
models have been underestimated in current measurements from lensing, and
need to be carefully reconsidered.Comment: Accepted for publication in Astronomy and Astrophysics. Discussion
expanded (MSD and velocity dispersion, MSD and free form lens models, MSD and
multiple source redshifts
Source-position transformation -- an approximate invariance in strong gravitational lensing
The main obstacle for gravitational lensing to determine accurate masses of
deflectors, or to determine precise estimates for the Hubble constant, is the
degeneracy of lensing observables with respect to the mass-sheet transformation
(MST). The MST is a global modification of the mass distribution which leaves
all image positions, shapes and flux ratios invariant, but which changes the
time delay. Here we show that another global transformation of lensing mass
distributions exists which almost leaves image positions and flux ratios
invariant, and of which the MST is a special case. Whereas for axi-symmetric
lenses this source position transformation exactly reproduces all strong
lensing observables, it does so only approximately for more general lens
situations. We provide crude estimates for the accuracy with which the
transformed mass distribution can reproduce the same image positions as the
original lens model, and present an illustrative example of its performance.
This new invariance transformation most likely is the reason why the same
strong lensing information can be accounted for with rather different mass
models.Comment: Submitted to Astronomy and Astrophysics. Comments welcome. 9 page
Alignment of quasar polarizations with large-scale structures
We have measured the optical linear polarization of quasars belonging to
Gpc-scale quasar groups at redshift z ~ 1.3. Out of 93 quasars observed, 19 are
significantly polarized. We found that quasar polarization vectors are either
parallel or perpendicular to the directions of the large-scale structures to
which they belong. Statistical tests indicate that the probability that this
effect can be attributed to randomly oriented polarization vectors is of the
order of 1%. We also found that quasars with polarization perpendicular to the
host structure preferentially have large emission line widths while objects
with polarization parallel to the host structure preferentially have small
emission line widths. Considering that quasar polarization is usually either
parallel or perpendicular to the accretion disk axis depending on the
inclination with respect to the line of sight, and that broader emission lines
originate from quasars seen at higher inclinations, we conclude that quasar
spin axes are likely parallel to their host large-scale structures.Comment: Accepted for publication in Astronomy and Astrophysic
The different origins of high- and low-ionization broad emission lines revealed by gravitational microlensing in the Einstein cross
We investigate the kinematics and ionization structure of the broad emission
line region of the gravitationally lensed quasar QSO2237+0305 (the Einstein
cross) using differential microlensing in the high- and low-ionization broad
emission lines. We combine visible and near-infrared spectra of the four images
of the lensed quasar and detect a large-amplitude microlensing effect
distorting the high-ionization CIV and low-ionization H line profiles
in image A. While microlensing only magnifies the red wing of the Balmer line,
it symmetrically magnifies the wings of the CIV emission line. Given that the
same microlensing pattern magnifies both the high- and low-ionization broad
emission line regions, these dissimilar distortions of the line profiles
suggest that the high- and low-ionization regions are governed by different
kinematics. Since this quasar is likely viewed at intermediate inclination, we
argue that the differential magnification of the blue and red wings of
H favors a flattened, virialized, low-ionization region whereas the
symmetric microlensing effect measured in CIV can be reproduced by an emission
line formed in a polar wind, without the need of fine-tuned caustic
configurations.Comment: 11 pages, 4 figures, A&
Large scale correlations of quasar polarisation vectors: Hints of extreme scale structures?
A survey measuring quasar polarization vectors has been started in two
regions towards the North and South Galactic Poles. Here, We review the
discovery of significant correlations of orientations of polarization vectors
over huge angular distances. We report new results including a larger sample of
the quasars confirming the existence of coherent orientations at redshifts z>1.Comment: Proc. of Astronomical Polarimetry - Current Status and Future
Directions March 15-19, 2004. Waikoloa Beach Marriott, Hawaii. 5 pages, 3
figure
HST observations of gravitationally lensed QSOs
Thanks to its sharp view, HST has significantly improved our knowledge of tens of gravitationally lensed quasars in four different respects: (1) confirming their lensed nature; (2) detecting the lensing galaxy responsible for the image splitting; (3) improving the astrometric accuracy on the positions of the unresolved QSO images and of the lens; (4) resolving extended lensed structures from the QSO hosts into faint NIR or optical rings or arcs. These observations have helped to break some degeneracies on the lens potential, to probe the galaxy evolution and to reconstruct the true shape of the QSO host with an increased angular resolution
Microlensing of the broad emission line region in the lensed quasar J1004+4112
J1004+4112 is a lensed quasar for which the first broad emission line profile
deformations due to microlensing were identified. Detailed interpretations of
these features have nevertheless remained controversial. Based on 15 spectra
obtained from 2003 to 2018, we revisit the microlensing effect that distorts
the CIV broad emission line profile. We show that the microlensing-induced line
profile distortions in image A, although variable, are remarkably similar over
a period of 15 years. They are characterized by a strong magnification of the
blue part of the line profile, a strong demagnification of the red part of the
line profile, and a small-to-negligible demagnification of the line core. We
used the microlensing effect to constrain the broad emission-line region (BLR)
size, geometry, and kinematics. For this purpose, we modeled the deformation of
the emission lines considering three simple, representative BLR models: a
Keplerian disk, an equatorial wind, and a biconical polar wind, with various
inclinations with respect to the line of sight. We find that the observed
magnification profile of the CIV emission line can be reproduced with the
simple BLR models we considered, without the need for more complex BLR
features. The magnification appears dominated by the position of the BLR with
respect to the caustic network -- and not by the velocity-dependent size of the
BLR. The favored models for the CIV BLR are either the Keplerian disk or the
equatorial wind, depending on the orientation of the BLR axis with respect to
the caustic network. We also find that the polar wind model can be discarded.
We measured the CIV BLR half-light radius as
light-days. This value is smaller than the BLR radius expected from the
radius-luminosity relation derived from reverberation mapping, but it is still
in reasonable agreement given the large uncertainties.Comment: Accepted for publication in Astronomy & Astrophysic
Modeling biases from constant stellar mass-to-light ratio assumption in galaxy dynamics and strong lensing
A constant stellar-mass to light ratio has been widely-used in
studies of galaxy dynamics and strong lensing, which aim at disentangling the
mass density distributions of dark matter and baryons. In this work, we take
early-type galaxies from the cosmological hydrodynamic IllustrisTNG-100
simulation to investigate possible systematic bias in the inferences due to a
constant assumption. To do so, we construct two-component matter
density models, where one component describes the dark matter distribution, the
other one for the stellar mass, which is made to follow the light profile by
assuming a constant factor of . Specifically, we adopt multiple
commonly used dark matter models and light distributions. We fit the
two-component models directly to the {\it total} matter density distributions
of simulated galaxies to eliminate systematics from other modelling procedures.
We find that galaxies in general have more centrally-concentrated stellar mass
profile than their light distribution. This is more significant among more
massive galaxies, for which the profile rises up markedly towards
the centre and may often exhibit a dented feature due to on-going star
formation at about one effective radius, encompassing a quenched bulge region.
As a consequence, a constant causes a model degeneracy to be
artificially broken under specific model assumptions, resulting in strong and
model-dependent biases on estimated properties, such as the central dark matter
fraction and the initial mass function. Either a steeper dark matter profile
with an over-predicted density fraction, or an over-predicted stellar mass
normalization () is often obtained through model fitting. The
exact biased behaviour depends on the slope difference between mass and light,
as well as on the adopted models for dark matter and light.Comment: 20 pages, 13 figure
Observations of radio-quiet quasars at 10mas resolution by use of gravitational lensing
We present VLA detections of radio emission in four four-image gravitational
lens systems with quasar sources: HS0810+2554, RXJ0911+0511, HE04351223 and
SDSSJ0924+0219, and e-MERLIN observations of two of the systems. The first
three are detected at a high level of significance, and SDSS J0924+0219 is
detected. HS0810+2554 is resolved, allowing us for the first time to achieve
10-mas resolution of the source frame in the structure of a radio quiet quasar.
The others are unresolved or marginally resolved. All four objects are among
the faintest radio sources yet detected, with intrinsic flux densities in the
range 1-5Jy; such radio objects, if unlensed, will only be observable
routinely with the Square Kilometre Array. The observations of HS0810+2554,
which is also detected with e-MERLIN, strongly suggest the presence of a
mini-AGN, with a radio core and milliarcsecond scale jet. The flux densities of
the lensed images in all but HE0435-1223 are consistent with smooth galaxy lens
models without the requirement for smaller-scale substructure in the model,
although some interesting anomalies are seen between optical and radio flux
densities. These are probably due to microlensing effects in the optical.Comment: Accepted by MNRA
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