33 research outputs found
A simple analysis of halo density profiles using gravitational lensing time delays
Gravitational lensing time delays depend upon the Hubble constant and the
density distribution of the lensing galaxies. This allows one to either model
the lens and estimate the Hubble constant, or to use a prior on the Hubble
constant from other studies and investigate what the preferred density
distribution is. Some studies have required compact dark matter halos (constant
M/L ratio) in order to reconcile gravitational lenses with the HST/WMAP value
of the Hubble constant (72 +/- 8 km/s /Mpc and 72 +/- 5 km/s /Mpc,
respectively). This is in direct contradiction with X-ray, stellar dynamical,
and weak lensing studies, which all point towards extended halos and isothermal
density profiles. In this work, we examine an up-to-date sample of 13 lensing
galaxies resulting in a data set consisting of 21 time delays. We select
systems in which there is a single primary lensing galaxy (e.g. excluding
systems undergoing mergers). Analysis is performed using analytic models based
upon a powerlaw density profile (rho \propto r^-n) of which the isothermal
profile is a special case (n = 2). This yields a value of n = 2.11+/-0.12
(3sigma) for the mean profile when modeling with a prior on the Hubble
constant, which is only consistent with isothermality within 3 sigma. Note that
this is a formal error from our calculations, and does not include the impact
of sample selection or simplifications in the lens modeling. We conclude that
time delays are a useful probe of density profiles, in particular as a function
of the environment in which the lens resides, when combined with a prior on the
Hubble constant.Comment: A&A accepte
The mass profile of early-type galaxies in overdense environments: the case of the double source plane gravitational lens SL2SJ02176-0513
SL2SJ02176-0513 is a remarkable lens for the presence of two multiply-imaged
systems at different redshifts lensed by a foreground massive galaxy at : a bright cusp arc at and an additional
double-image system at an estimated redshift of based on
photometry and lensing geometry. The system is located about 400 kpc away from
the center of a massive group of galaxies. Mass estimates for the group are
available from X-ray observations and satellite kinematics. Multicolor
photometry provides an estimate of the stellar mass of the main lens galaxy.
The lensing galaxy is modeled with two components (stars and dark matter), and
we include the perturbing effect of the group environment, and all available
constraints. We find that classic lensing degeneracies, e.g. between external
convergence and mass density slope, are significantly reduced with respect to
standard systems and infer tight constraints on the mass density profile: (i)
the dark matter content of the main lens galaxy is in line with that of typical
galaxies ; (ii) the required mass
associated with the dark matter halo of the nearby group is consistent with
X-ray and weak-lensing estimates (); (iii)
accounting for the group contribution in the form of an external convergence,
the slope of the mass density profile of the main lens galaxy alone is found to
be , consistent with the isothermal ()
slope. We demonstrate that multiple source plane systems together with good
ancillary dataset can be used to disentangle local and environmental effects.Comment: 10 pages, 6 figures, submitted to A&
Estimating cosmological parameters from future gravitational lens surveys
Upcoming ground and space based observatories such as the DES, the LSST, the
JDEM concepts and the SKA, promise to dramatically increase the size of strong
gravitational lens samples. A significant fraction of the systems are expected
to be time delay lenses. Many of the existing lensing degeneracies become less
of an issue with large samples since the distributions of a number of
parameters are predictable, and can be incorporated into an analysis, thus
helping to lessen the degeneracy. Assuming a mean galaxy density profile that
does not evolve with redshift, a Lambda-CDM cosmology, and Gaussian
distributions for bulk parameters describing the lens and source populations,
we generate synthetic lens catalogues and examine the relationship between
constraints on the Omega_m - Omega_Lambda plane and H_0 with increasing lens
sample size. We find that, with sample sizes of ~400 time delay lenses, useful
constraints can be obtained for Omega_m and Omega_Lambda with approximately
similar levels of precision as from the best of other methods. In addition,
sample sizes of ~100 time delay systems yield estimates of H_0 with errors of
only a couple of percent, exceeding the level of precision from current best
estimates such as the HST Key Project. We note that insufficient prior
knowledge of the lens samples employed in the analysis, via under or
overestimates in the mean values of the sample distributions, results in
broadening of constraints. This highlights the need for sound prior knowledge
of the sample before useful cosmological constraints can be obtained from large
time delay samples (abridged).Comment: 10 pages, 3 figures, 1 table. Accepted for publication in MNRA
The Millennium Run Observatory: First Light
Simulations of galaxy evolution aim to capture our current understanding as
well as to make predictions for testing by future experiments. Simulations and
observations are often compared in an indirect fashion: physical quantities are
estimated from the data and compared to models. However, many applications can
benefit from a more direct approach, where the observing process is also
simulated and the models are seen fully from the observer's perspective. To
facilitate this, we have developed the Millennium Run Observatory (MRObs), a
theoretical virtual observatory which uses virtual telescopes to `observe'
semi-analytic galaxy formation models based on the suite of Millennium Run dark
matter simulations. The MRObs produces data that can be processed and analyzed
using the standard software packages developed for real observations. At
present, we produce images in forty filters from the rest-frame UV to IR for
two stellar population synthesis models, three different models of IGM
absorption, and two cosmologies (WMAP1/7). Galaxy distributions for a large
number of mock lightcones can be `observed' using models of major ground- and
space-based telescopes. The data include lightcone catalogues linked to
structural properties of galaxies, pre-observation model images, mock telescope
images, and Source Extractor products that can all be traced back to the higher
level dark matter, semi-analytic galaxy, and lightcone catalogues available in
the Millennium database. Here, we describe our methods and announce a first
public release of simulated surveys (e.g., SDSS, CFHT-LS, GOODS, GOODS/ERS,
CANDELS, and HUDF). The MRObs browser, an online tool, further facilitates
exploration of the simulated data. We demonstrate the benefits of a direct
approach through a number of example applications (galaxy number counts in
CANDELS, clusters, morphologies, and dropout selections).Comment: MNRAS, in press. Millennium Run Observatory data products, online
tools, and more available through http://galformod.mpa-garching.mpg.de/mrobs
The Hubble Constant
I review the current state of determinations of the Hubble constant, which
gives the length scale of the Universe by relating the expansion velocity of
objects to their distance. There are two broad categories of measurements. The
first uses individual astrophysical objects which have some property that
allows their intrinsic luminosity or size to be determined, or allows the
determination of their distance by geometric means. The second category
comprises the use of all-sky cosmic microwave background, or correlations
between large samples of galaxies, to determine information about the geometry
of the Universe and hence the Hubble constant, typically in a combination with
other cosmological parameters. Many, but not all, object-based measurements
give values of around 72-74km/s/Mpc , with typical errors of 2-3km/s/Mpc.
This is in mild discrepancy with CMB-based measurements, in particular those
from the Planck satellite, which give values of 67-68km/s/Mpc and typical
errors of 1-2km/s/Mpc. The size of the remaining systematics indicate that
accuracy rather than precision is the remaining problem in a good determination
of the Hubble constant. Whether a discrepancy exists, and whether new physics
is needed to resolve it, depends on details of the systematics of the
object-based methods, and also on the assumptions about other cosmological
parameters and which datasets are combined in the case of the all-sky methods.Comment: Extensively revised and updated since the 2007 version: accepted by
Living Reviews in Relativity as a major (2014) update of LRR 10, 4, 200
Strong gravitational lensing probes of the particle nature of dark matter
There is a vast menagerie of plausible candidates for the constituents of
dark matter, both within and beyond extensions of the Standard Model of
particle physics. Each of these candidates may have scattering (and other)
cross section properties that are consistent with the dark matter abundance,
BBN, and the most scales in the matter power spectrum; but which may have
vastly different behavior at sub-galactic "cutoff" scales, below which dark
matter density fluctuations are smoothed out. The only way to quantitatively
measure the power spectrum behavior at sub-galactic scales at distances beyond
the local universe, and indeed over cosmic time, is through probes available in
multiply imaged strong gravitational lenses. Gravitational potential
perturbations by dark matter substructure encode information in the observed
relative magnifications, positions, and time delays in a strong lens. Each of
these is sensitive to a different moment of the substructure mass function and
to different effective mass ranges of the substructure. The time delay
perturbations, in particular, are proving to be largely immune to the
degeneracies and systematic uncertainties that have impacted exploitation of
strong lenses for such studies. There is great potential for a coordinated
theoretical and observational effort to enable a sophisticated exploitation of
strong gravitational lenses as direct probes of dark matter properties. This
opportunity motivates this white paper, and drives the need for: a) strong
support of the theoretical work necessary to understand all astrophysical
consequences for different dark matter candidates; and b) tailored
observational campaigns, and even a fully dedicated mission, to obtain the
requisite data.Comment: Science white paper submitted to the Astro2010 Decadal Cosmology &
Fundamental Physics Science Frontier Pane