919 research outputs found
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
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
Detection of microgauss coherent magnetic fields in a galaxy five billion years ago
Magnetic fields play a pivotal role in the physics of interstellar medium in
galaxies, but there are few observational constraints on how they evolve across
cosmic time. Spatially resolved synchrotron polarization maps at radio
wavelengths reveal well-ordered large-scale magnetic fields in nearby galaxies
that are believed to grow from a seed field via a dynamo effect. To directly
test and characterize this theory requires magnetic field strength and geometry
measurements in cosmologically distant galaxies, which are challenging to
obtain due to the limited sensitivity and angular resolution of current radio
telescopes. Here, we report the cleanest measurements yet of magnetic fields in
a galaxy beyond the local volume, free of the systematics traditional
techniques would encounter. By exploiting the scenario where the polarized
radio emission from a background source is gravitationally lensed by a
foreground galaxy at z = 0.439 using broadband radio polarization data, we
detected coherent G magnetic fields in the lensing disk galaxy as seen 4.6
Gyrs ago, with similar strength and geometry to local volume galaxies. This is
the highest redshift galaxy whose observed coherent magnetic field property is
compatible with a mean-field dynamo origin.Comment: 29 pages, 5 figures (including Supplementary Information). Published
in Nature Astronomy on August 28, 201
- …