226 research outputs found
Statistical and systematic uncertainties in pixel-based source reconstruction algorithms for gravitational lensing
Gravitational lens modeling of spatially resolved sources is a challenging
inverse problem with many observational constraints and model parameters. We
examine established pixel-based source reconstruction algorithms for de-lensing
the source and constraining lens model parameters. Using test data for four
canonical lens configurations, we explore statistical and systematic
uncertainties associated with gridding, source regularisation, interpolation
errors, noise, and telescope pointing. Specifically, we compare two gridding
schemes in the source plane: a fully adaptive grid that follows the lens
mapping but is irregular, and an adaptive Cartesian grid. We also consider
regularisation schemes that minimise derivatives of the source (using two
finite difference methods) and introduce a scheme that minimises deviations
from an analytic source profile. Careful choice of gridding and regularisation
can reduce "discreteness noise" in the surface that is inherent in the
pixel-based methodology. With a gridded source, some degree of interpolation is
unavoidable, and errors due to interpolation need to be taken into account
(especially for high signal-to-noise data). Different realisations of the noise
and telescope pointing lead to slightly different values for lens model
parameters, and the scatter between different "observations" can be comparable
to or larger than the model uncertainties themselves. The same effects create
scatter in the lensing magnification at the level of a few percent for a peak
signal-to-noise ratio of 10, which decreases as the data quality improves.Comment: 20 pages, 18 figures, accepted to MNRAS, see
http://physics.rutgers.edu/~tagoreas/papers/ for high resolution image
Biases in inferring dark matter profiles from dynamical and lensing measurements
The degeneracy between disc and halo contributions in spiral galaxy rotation
curves makes it difficult to obtain a full understanding of the distribution of
baryons and dark matter in disc galaxies like our own Milky Way. Using mock
data, we study how constraints on dark matter profiles obtained from
kinematics, strong lensing, or a combination of the two are affected by
assumptions about the halo model. We compare four different models: spherical
isothermal and Navarro-Frenk-White halos, along with spherical and elliptical
Burkert halos. For both kinematics and lensing we find examples where different
models fit the data well but give enclosed masses that are inconsistent with
the true (i.e., input) values. This is especially notable when the input and
fit models differ in having cored or cuspy profiles (such as fitting an NFW
model when the underlying dark matter distribution follows a different
profile). We find that mass biases are more pronounced with lensing than with
kinematics, and using both methods can help reduce the bias and provide
stronger constraints on the dark matter distributions.Comment: 13 pages, 8 figures, submitted for MNRAS publication Nov 15th 201
The Impact of Baryonic Cooling on Giant Arc Abundances
Using ray tracing for simple analytic profiles, we demonstrate that the
lensing cross section for producing giant arcs has distinct contributions due
to arcs formed through image distortion only, and arcs form from the merging of
two or three images. We investigate the dependence of each of these
contributions on halo ellipticity and on the slope of the density profile, and
demonstrate that at fixed Einstein radius, the lensing cross section increases
as the halo profile becomes steeper. We then compare simulations with and
without baryonic cooling of the same cluster for a sample of six clusters, and
demonstrate that cooling can increase the overall abundance of giant arcs by
factors of a few. The net boost to the lensing probability for individual
clusters is mass dependent, and can lower the effective low mass limit of
lensing clusters. This last effect can potentially increase the number of
lensing clusters by an extra 50%. While the magnitude of these effects may be
overestimated due to the well known overcooling problem in simulations, it is
evident that baryonic cooling has a non-negligible impact on the expected
abundance of giant arcs, and hence cosmological constraints from giant arc
abundances may be subject to large systematic errors.Comment: ApJ Submitte
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