785 research outputs found
A New Hybrid Framework to Efficiently Model Lines of Sight to Gravitational Lenses
In strong gravitational lens systems, the light bending is usually dominated
by one main galaxy, but may be affected by other mass along the line of sight
(LOS). Shear and convergence can be used to approximate the contributions from
less significant perturbers (e.g. those that are projected far from the lens or
have a small mass), but higher order effects need to be included for objects
that are closer or more massive. We develop a framework for multiplane lensing
that can handle an arbitrary combination of tidal planes treated with shear and
convergence and planes treated exactly (i.e., including higher order terms).
This framework addresses all of the traditional lensing observables including
image positions, fluxes, and time delays to facilitate lens modelling that
includes the non-linear effects due to mass along the LOS. It balances accuracy
(accounting for higher-order terms when necessary) with efficiency (compressing
all other LOS effects into a set of matrices that can be calculated up front
and cached for lens modelling). We identify a generalized multiplane mass sheet
degeneracy, in which the effective shear and convergence are sums over the
lensing planes with specific, redshift-dependent weighting factors.Comment: 13 pages, 2 figure
Optimal Mass Configurations for Lensing High-Redshift Galaxies
We investigate the gravitational lensing properties of lines of sight
containing multiple cluster-scale halos, motivated by their ability to lens
very high-redshift (z ~ 10) sources into detectability. We control for the
total mass along the line of sight, isolating the effects of distributing the
mass among multiple halos and of varying the physical properties of the halos.
Our results show that multiple-halo lines of sight can increase the magnified
source-plane region compared to the single cluster lenses typically targeted
for lensing studies, and thus are generally better fields for detecting very
high-redshift sources. The configurations that result in optimal lensing cross
sections benefit from interactions between the lens potentials of the halos
when they overlap somewhat on the sky, creating regions of high magnification
in the source plane not present when the halos are considered individually. The
effect of these interactions on the lensing cross section can even be
comparable to changing the total mass of the lens from 10^15 M_sun to 3x10^15
M_sun. The gain in lensing cross section increases as the mass is split into
more halos, provided that the lens potentials are projected close enough to
interact with each other. A nonzero projected halo angular separation, equal
halo mass ratio, and high projected halo concentration are the best mass
configurations, whereas projected halo ellipticity, halo triaxiality, and the
relative orientations of the halos are less important. Such high mass,
multiple-halo lines of sight exist in the SDSS.Comment: Accepted for publication in ApJ; emulateapj format; 24 pages, 13
figures, 1 table; plots updated to reflect erratu
Microlensing of Central Images in Strong Gravitational Lens Systems
We study microlensing of the faint images that form close to the centers of
strong gravitational lens galaxies. These central images, which have finally
begun to yield to observations, naturally appear in dense stellar fields and
may be particularly sensitive to fine granularity in the mass distribution. The
microlensing magnification maps for overfocussed (i.e., demagnified) images
differ strikingly from those for magnified images. In particular, the familiar
"fold" and "cusp" features of maps for magnified images are only present for
certain values of the fraction, f, of the surface mass density contained in
stars. For central images, the dispersion in microlensing magnifications is
generally larger than for normal (minimum and saddle) images, especially when
the source is comparable to or larger than the stellar Einstein radius. The
dispersion depends in a complicated way on f; this behaviour may hold the key
to using microlensing as a probe of the relative densities of stars and dark
matter in the cores of distant galaxies. Quantitatively, we predict that the
central image C in PMN J1632-0033 has a magnification dispersion of 0.6
magnitudes for Rsrc/Rein <~ 1, or 0.3 mag for Rsrc/Rein = 10. For comparison,
the dispersions are 0.5-0.6 mag for image B and 0.05-0.1 mag for image A, if
Rsrc/Rein <~ 1; and just 0.1 mag for B and 0.008 mag for A if Rsrc/Rein = 10.
(The dispersions can be extrapolated to larger sources sizes as sigma \propto
Rsrc^{-1}.) Thus, central images are more susceptible than other lensed images
to microlensing and hence good probes for measuring source sizes.Comment: 12 pages; accepted in MNRAS; many new magnification maps and
significantly expanded analysis of magnification map structur
The Effect of Environment on Shear in Strong Gravitational Lenses
Using new photometric and spectroscopic data in the fields of nine strong
gravitational lenses that lie in galaxy groups, we analyze the effects of both
the local group environment and line-of-sight galaxies on the lens potential.
We use Monte Carlo simulations to derive the shear directly from measurements
of the complex lens environment, providing the first detailed independent check
of the shear obtained from lens modeling. We account for possible tidal
stripping of the group galaxies by varying the fraction of total mass
apportioned between the group dark matter halo and individual group galaxies.
The environment produces an average shear of gamma = 0.08 (ranging from 0.02 to
0.17), significant enough to affect quantities derived from lens observables.
However, the direction and magnitude of the shears do not match those obtained
from lens modeling in three of the six 4-image systems in our sample (B1422,
RXJ1131, and WFI2033). The source of this disagreement is not clear, implying
that the assumptions inherent in both the environment and lens model approaches
must be reconsidered. If only the local group environment of the lens is
included, the average shear is gamma = 0.05 (ranging from 0.01 to 0.14),
indicating that line-of-sight contributions to the lens potential are not
negligible. We isolate the effects of various theoretical and observational
uncertainties on our results. Of those uncertainties, the scatter in the
Faber-Jackson relation and error in the group centroid position dominate.
Future surveys of lens environments should prioritize spectroscopic sampling of
both the local lens environment and objects along the line of sight,
particularly those bright (I < 21.5) galaxies projected within 5' of the lens.Comment: Accepted for publication in The Astrophysical Journal; 28 pages, 9
figures, 5 table
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