74 research outputs found
Constraints on the Inner Mass Profiles of Lensing Galaxies from Missing Odd Images
Most gravitational lens systems consist of two or four observable images. The
absence of detectable odd images allows us to place a lower limit on the
power-law slope of the inner mass profile of lensing galaxies. Using a sample
of six two-image radio-loud lens systems and assuming a singular power-law
surface density (Sigma proportional to r^{-beta}) for the inner several kpc of
the mass distribution, we find that there is less than a 10% probability that
the data are consistent with profile slopes beta < 0.80. Furthermore,
individual mass modeling yields beta > 0.85 for B0739+366 and beta > 0.91 for
B1030+074. Modeling central black holes as additional point masses changes the
constraints in these systems to beta > 0.84 and beta > 0.83, respectively. The
inner mass profiles of lensing galaxies are therefore not much shallower than
isothermal.Comment: Final published version, minor typos corrected, 13 page
Efficient Solution of Rational Conics
We present efficient algorithms for solving Legendre equations over Q (equivalently, for finding rational points on rational conics) and parametrizing all solutions. Unlike existing algorithms, no integer factorization is required, provided that the prime factors of the discriminant are known
Measuring Supermassive Black Holes in Distant Galaxies with Central Lensed Images
The supermassive black hole at the center of a distant galaxy can be weighed,
in rare but realistic cases, when the galaxy acts as a strong gravitational
lens. The central image that should be produced by the lens is either destroyed
or accompanied by a second central image, depending on the mass of the black
hole. We demonstrate that when a central image pair is detected, the mass of
the black hole can be determined with an accuracy of < 0.1 dex, if the form of
the smooth mass distribution near the galaxy core is known. Uncertainty in the
central mass distribution introduces a systematic error in the black hole mass
measurement. However, even with nearly complete ignorance of the inner mass
distribution, the black hole mass can still be determined to within a factor of
10. Central image pairs should be readily observable with future radio
interferometers, allowing this technique to be used for a census of
supermassive black holes in inactive galaxies at significant redshift (0.2 < z
< 1.0).Comment: 12 pages including 4 figures, ApJL in press (edited to match
published version, one typo corrected
Galaxy Groups Associated with Gravitational Lenses and H_0 from B1608+656
Compact groups of galaxies recently have been discovered in association with
several strong gravitational lens systems. These groups provide additional
convergence to the lensing potential and thus affect the value of H_0 derived
from the systems. Lens system time delays are now being measured with
uncertainties of only a few percent or better. Additionally, vast improvements
are being made in incorporating observational constraints such as Einstein ring
structures and stellar velocity dispersions into the lens models. These
advances are reducing the uncertainties on H_0 to levels at which the the
effects of associated galaxy groups may contribute significantly to the overall
error budget. We describe a dedicated multiwavelength program, using Keck, HST,
and Chandra, to find such groups and measure their properties. We present, as a
case study, results obtained from observations of the CLASS lens system
B1608+656 and discuss the implications for the value of H_0 derived from this
system.Comment: To appear in Proceedings of IAU Symposium 225: Impact of
Gravitational Lensing on Cosmology, 6 pages, 2 figure
Strong Gravitational Lensing and Dark Energy
We investigate the statistics of gravitational lenses in flat, low-density
cosmological models with different cosmic equations of state w. We compute the
lensing probabilities as a function of image separation \theta using a lens
population described by the mass function of Jenkins et al. and modeled as
singular isothermal spheres on galactic scales and as Navarro, Frenk & White
halos on cluster scales. It is found that COBE-normalized models with w > - 0.4
produce too few arcsecond-scale lenses in comparison with the JVAS/CLASS radio
survey, a result that is consistent with other observational constraints on w.
The wide-separation (\theta > 4'') lensing rate is a particularly sensitive
probe of both w and the halo mass concentration. The absence of these systems
in the current JVAS/CLASS data excludes highly concentrated halos in w < -0.7
models. The constraints can be improved by ongoing and future lensing surveys
of > 10^5 sources.Comment: 12 pages, 3 figures. Accepted for publication in ApJ Letter
The central image of a gravitationally lensed quasar
A galaxy can act as a gravitational lens, producing multiple images of a
background object. Theory predicts there should be an odd number of images but,
paradoxically, almost all observed lenses have 2 or 4 images. The missing image
should be faint and appear near the galaxy's center. These ``central images''
have long been sought as probes of galactic cores too distant to resolve with
ordinary observations. There are five candidates, but in one case the third
image is not necessarily a central image, and in the others, the central
component might be a foreground source rather than a lensed image. Here we
report the most secure identification of a central image, based on radio
observations of PMN J1632-0033, one of the latter candidates. Lens models
incorporating the central image show that the mass of the lens galaxy's central
black hole is less than 2 x 10^8 M_sun, and the galaxy's surface density at the
location of the central image is more than 20,000 M_sun per square parsec, in
agreement with expectations based on observations of galaxies hundreds of times
closer to the Earth.Comment: Nature, in press [7 pp, 2 figs]. Standard media embargo applies
before publicatio
New Constraints on the Complex Mass Substructure in Abell 1689 from Gravitational Flexion
In a recent publication, the flexion aperture mass statistic was found to
provide a robust and effective method by which substructure in galaxy clusters
might be mapped. Moreover, we suggested that the masses and mass profile of
structures might be constrained using this method. In this paper, we apply the
flexion aperture mass technique to HST ACS images of Abell 1689. We demonstrate
that the flexion aperture mass statistic is sensitive to small-scale structures
in the central region of the cluster. While the central potential is not
constrained by our method, due largely to missing data in the central
0.5 of the cluster, we are able to place constraints on the masses and
mass profiles of prominent substructures. We identify 4 separate mass peaks,
and use the peak aperture mass signal and zero signal radius in each case to
constrain the masses and mass profiles of these substructures. The three most
massive peaks exhibit complex small-scale structure, and the masses indicated
by the flexion aperture mass statistic suggest that these three peaks represent
the dominant substructure component of the cluster (). Their complex structure indicates that the cluster --
far from being relaxed -- may have recently undergone a merger. The smaller,
subsidiary peak is located coincident with a group of galaxies within the
cluster, with mass . These results are in
excellent agreement with previous substructure studies of this cluster.Comment: 18 pages, 10 figures, MNRAS accepted (7 Dec 2010
The Sloan Digital Sky Survey Quasar Lens Search. IV. Statistical Lens Sample from the Fifth Data Release
We present the second report of our systematic search for strongly lensed
quasars from the data of the Sloan Digital Sky Survey (SDSS). From extensive
follow-up observations of 136 candidate objects, we find 36 lenses in the full
sample of 77,429 spectroscopically confirmed quasars in the SDSS Data Release
5. We then define a complete sample of 19 lenses, including 11 from our
previous search in the SDSS Data Release 3, from the sample of 36,287 quasars
with i<19.1 in the redshift range 0.6<z<2.2, where we require the lenses to
have image separations of 1"<\theta<20" and i-band magnitude differences
between the two images smaller than 1.25 mag. Among the 19 lensed quasars, 3
have quadruple-image configurations, while the remaining 16 show double images.
This lens sample constrains the cosmological constant to be
\Omega_\Lambda=0.84^{+0.06}_{-0.08}(stat.)^{+0.09}_{-0.07}(syst.) assuming a
flat universe, which is in good agreement with other cosmological observations.
We also report the discoveries of 7 binary quasars with separations ranging
from 1.1" to 16.6", which are identified in the course of our lens survey. This
study concludes the construction of our statistical lens sample in the full
SDSS-I data set.Comment: 37 pages, 2 figures and 5 tables, accepted to A
Why is the Fraction of Four-Image Radio Lens Systems So High?
We investigate the frequency of two- and four-image gravitational lens
systems in the Jodrell-VLA Astrometric Survey (JVAS) and Cosmic Lens All-Sky
Survey (CLASS), and the possible implications for dark matter halo properties.
A simple lensing statistics model, which describes lens galaxies as singular
isothermal ellipsoids with a projected axis ratio distribution derived from the
surface brightness ellipticities of early-type galaxies in the Coma cluster, is
ruled out at the 98% level since it predicts too few four-image lenses (quads).
We consider a range of factors that may be increasing the frequency of radio
quads, including external shear fields, mass distributions flatter than the
light, shallow lensing mass profiles, finite core radii, satellite galaxies,
and alterations to the luminosity function for faint flat-spectrum radio
sources. We find that none of these mechanisms provide a compelling solution to
the quad problem on their own while remaining consistent with other
observational constraints.Comment: Final version. 27 pages, including 9 figs, minor typos corrected, ApJ
in press (June 2001
Neurocognition with maraviroc compared with tenofovir in HIV
To determine whether maraviroc (MVC) has unique neurocognitive benefits in the context of initial antiretroviral therapy (ART)
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