426 research outputs found
CFHT AO Imaging of the CLASS Gravitational Lens System B1359+154
We present adaptive optics imaging of the CLASS gravitational lens system
B1359+154 obtained with the Canada-France-Hawaii Telescope (CFHT) in the
infrared K band. The observations show at least three brightness peaks within
the ring of lensed images, which we identify as emission from multiple lensing
galaxies. The results confirm the suspected compound nature of the lens, as
deduced from preliminary mass modeling. The detection of several additional
nearby galaxies suggests that B1359+154 is lensed by the compact core of a
small galaxy group. We attempted to produce an updated lens model based on the
CFHT observations and new 5-GHz radio data obtained with the MERLIN array, but
there are too few constraints to construct a realistic model at this time. The
uncertainties inherent with modeling compound lenses make B1359+154 a
challenging target for Hubble constant determination through the measurement of
differential time delays. However, time delays will offer additional
constraints to help pin down the mass model. This lens system therefore
presents a unique opportunity to directly measure the mass distribution of a
galaxy group at intermediate redshift.Comment: 12 pages including 3 figures; ApJL accepte
Can Strong Gravitational Lensing Constrain Dark Energy?
We discuss the ratio of the angular diameter distances from the source to the
lens, , and to the observer at present, , for various dark
energy models. It is well known that the difference of s between the
models is apparent and this quantity is used for the analysis of Type Ia
supernovae. However we investigate the difference between the ratio of the
angular diameter distances for a cosmological constant,
and that for other dark energy models,
in this paper. It has been known that there is
lens model degeneracy in using strong gravitational lensing. Thus, we
investigate the model independent observable quantity, Einstein radius
(), which is proportional to both and velocity
dispersion squared, . values depend on the parameters
of each dark energy model individually. However, for the various dark energy models, is well within
the error of for most of the parameter spaces of the dark energy
models. Thus, a single strong gravitational lensing by use of the Einstein
radius may not be a proper method to investigate the property of dark energy.
However, better understanding to the mass profile of clusters in the future or
other methods related to arc statistics rather than the distances may be used
for constraints on dark energy.Comment: 15 pages, 13 figures, Accepted in PR
A determination of H_0 with the CLASS gravitational lens B1608+656: II. Mass models and the Hubble constant from lensing
EDITED FROM PAPER: We present mass models of the four-image gravitational
lens system B1608+656. A mass model for the lens galaxies has been determined
that reproduces the image positions, two out of three flux-density ratios and
the model time delays.
Using the time delays determined by Fassnacht et al. (1999a), we find that
the best isothermal mass model gives H_0=59^{+7}_{-6} km/s/Mpc for Omega_m=1
and Omega_l=0.0, or H_0=(65-63)^{+7}_{-6} km/s/Mpc for Omega_m=0.3 and Omega_l
= 0.0-0.7 (95.4% statistical confidence). A systematic error of +/-15 km/s/Mpc
is estimated.
This cosmological determination of H_0 agrees well with determinations from
three other gravitational lens systems (i.e. B0218+357, Q0957+561 and
PKS1830-211), SNe Ia, the S-Z effect and local determinations. The current
agreement on H_0 from four out of five gravitational lens systems (i)
emphasizes the reliability of its determination from isolated gravitational
lens systems and (ii) suggests that a close-to-isothermal mass profile can
describe disk galaxies, ellipticals and central cluster ellipticals.
The average of H_0 from B0218+357, Q0957+561, B1608+656 and PKS1830-211,
gives H_0(GL)=69 +/-7 km/s/Mpc for a flat universe with Omega_m=1 or H_0(GL)=74
+/-8 km/s/Mpc for Omega_m=0.3 and Omega_l=0.0-0.7. When including PG1115+080,
these values decrease to 64 +/-11 km/s/Mpc and 68 +/-13 km/s/Mpc (2-sigma
errors), respectively.Comment: Accepted for publication in ApJ. 34 pages, 4 figure
Testing a new analytic model for gravitational lensing probabilities
We study gravitational lensing with a multiple lens plane approach, proposing
a simple analytical model for the probability distribution function (PDF) of
the dark matter convergence, kappa, for the different lens planes in a given
cosmology as a function of redshift and smoothing angle, theta. The model is
fixed solely by the variance of kappa, which in turn is fixed by the amplitude
of the power spectrum, sigma_8. We test the PDF against a high resolution
Tree-Particle-Mesh simulation and find that it is far superior to the Gaussian
or the lognormal, especially for small values of theta << 1 arcmin and at large
values of kappa relevant to strong lensing. With this model, we predict the
probabilities of strong lensing by a single plane or by multiple planes. We
find that for theta ~ 10 arcsec, a single plane accounts for almost all (~ 98%)
of the strong lensing cases for source redshift unity. However, for a more
typical source redshift of 4, about 12% of the strong lensing cases will result
from the contribution of a secondary clump of matter along the line of sight,
introducing a systematic error in the determination of the surface density of
clusters, typically overestimating it by about 2-5%. We also find that matter
inhomogenieties introduce a dispersion in the value of the angular diameter
distance about its cosmological mean. The probable error relative to the mean
increases with redshift to a value of about 8% for z ~ 6 and theta ~ 10 arcsec.Comment: Accepted for publication in ApJ, 13 pages, 12 figures, revised
version, references added, section 6 expande
MERLIN/VLA imaging of the gravitational lens system B0218+357
Gravitational lenses offer the possibility of accurately determining the
Hubble parameter (H_0) over cosmological distances, and B0218+357 is one of the
most promising systems for an application of this technique. In particular this
system has an accurately measured time delay (10.5+/-0.4 d; Biggs et al. 1999)
and preliminary mass modelling has given a value for H_0 of 69 +13/-19
km/s/Mpc. The error on this estimate is now dominated by the uncertainty in the
mass modelling. As this system contains an Einstein ring it should be possible
to constrain the model better by imaging the ring at high resolution. To
achieve this we have combined data from MERLIN and the VLA at a frequency of 5
GHz. In particular MERLIN has been used in multi-frequency mode in order to
improve substantially the aperture coverage of the combined data set. The
resulting map is the best that has been made of the ring and contains many new
and interesting features. Efforts are currently underway to exploit the new
data for lensing constraints using the LensClean algorithm (Kochanek & Narayan
1992).Comment: Accepted for publication in MNRAS. 6 pages, 4 included PostScript
figure
Shear and Ellipticity in Gravitational Lenses
Galaxies modeled as singular isothermal ellipsoids with an axis ratio
distribution similar to the observed axis ratio distribution of E and S0
galaxies are statistically consistent with both the observed numbers of
two-image and four-image lenses and the inferred ellipticities of individual
lenses. However, no four-image lens is well fit by the model (typical
), the axis ratio of the model can be significantly
different from that of the observed lens galaxy, and the major axes of the
model and the galaxy may be slightly misaligned. We found that models with a
second, independent, external shear axis could fit the data well (typical
), while adding the same number of extra parameters to
the radial mass distribution does not produce such a dramatic improvement in
the fit. An independent shear axis can be produced by misalignments between the
luminous galaxy and its dark matter halo, or by external shear perturbations
due to galaxies and clusters correlated with the primary lens or along the line
of sight. We estimate that the external shear perturbations have no significant
effect on the expected numbers of two-image and four-image lenses, but that
they can be important perturbations in individual lens models. However, the
amplitudes of the external shears required to produce the good fits are larger
than our estimates for typical external shear perturbations (10-15% shear
instead of 1-3% shear) suggesting that the origin of the extra angular
structure must be intrinsic to the primary lens galaxy in most cases.Comment: 38 pages, 9 figures, submitted to Ap
Corporate Governance, Opaque Bank Activities, and Risk/Return Efficiency: Pre- and Post-Crisis Evidence from Turkey
Does better corporate governance unambiguously improve the risk/return efficiency of banks? Or does either a re-orientation of banks' revenue mix towards more opaque products, an economic downturn, or tighter supervision create off-setting or reinforcing effects? The authors relate bank efficiency to shortfalls from a stochastic risk/return frontier. They analyze how internal governance mechanisms (CEO duality, board experience, political connections, and education profile) and external governance mechanisms (discipline exerted by shareholders, depositors, or skilled employees) determine efficiency in a sample of Turkish banks. The 2000 financial crisis was a wakeup call for bank efficiency and corporate governance. As a result, better corporate governance mechanisms have been able to improve risk/return efficiency when the economic, regulatory, and supervisory environments are more stable and bank products are more complex.corporate governance;bank risk;noninterest income;crisis;frontier
Subaru Spectroscopy of the Gravitational Lens HST 14176+5226: Implications for a Large Cosmological Constan
We present new optical spectroscopy of the lens elliptical galax in the
``Einstein Cross'' lens system HST 14176+5226, using the Faint Object Camera
and Spectrograph (FOCAS) of the Subaru t Our spectroscopic observations are
aimed at measuring the stella dispersion of the lens galaxy, located at high
redshift of z_L= as an important component to lens models. We have measured
this 230 +- 14 km s^{-1} (1 sigma) inside 0.35 effective radi based on the
comparison between the observed galaxy spectrum and templates of three G-K
giants by means of the Fourier cross-corr To extract the significance of this
information on the geometry universe which also affects the lensing of the
background image, to fit three different lens models to the available data of
the Provided that the lens galaxy has the structural and dynamical p (i.e., its
radial density profile, core radius, and velocity ani similar to those of local
elliptical galaxies, we calculate the function for the simultaneous
reproduction of both the observed and newly measured velocity dispersion of the
lens. Although the interval depends rather sensitively on the adopted lens
models o parameters, our experiments suggest the larger likelihood for a
cosmological constant, Omega_Lambda: formal 1 sigma lower Omega_Lambda in the
flat universe ranges 0.73 to 0.97, where lower limit is basically unavailable.
This method for determinin model is thus dependent on lens models but is
insensitive to oth ambiguities, such as the dust absorption or the evolutionary
eff galaxies. Exploring spectroscopic observations of more lens gala redshift
may minimize the model uncertainties and thus place a m constraint on
Omega_Lambda.Comment: 23 pages, 5 figures, accepted for publication in The Astronomical
Journa
Effects of Ellipticity and Shear on Gravitational Lens Statistics
We study the effects of ellipticity in lens galaxies and external tidal shear
from neighboring objects on the statistics of strong gravitational lenses. For
isothermal lens galaxies normalized so that the Einstein radius is independent
of ellipticity and shear, ellipticity {\it reduces} the lensing cross section
slightly, and shear leaves it unchanged. Ellipticity and shear can
significantly enhance the magnification bias, but only if the luminosity
function of background sources is steep. Realistic distributions of ellipticity
and shear {\it lower} the total optical depth by a few percent for most source
luminosity functions, and increase the optical depth only for steep luminosity
functions. The boost in the optical depth is noticeable (>5%) only for surveys
limited to the brightest quasars (L/L_* > 10). Ellipticity and shear broaden
the distribution of lens image separations but do not affect the mean.
Ellipticity and shear naturally increase the abundance of quadruple lenses
relative to double lenses, especially for steep source luminosity functions,
but the effect is not enough (by itself) to explain the observed
quadruple-to-double ratio. With such small changes to the optical depth and
image separation distribution, ellipticity and shear have a small effect on
cosmological constraints from lens statistics: neglecting the two leads to
biases of just Delta Omega_M = 0.00 \pm 0.01 and Delta Omega_Lambda = -0.02 \pm
0.01 (where the errorbars represent statistical uncertainties in our
calculations).Comment: Optical depth normalization discussed. Matches the published versio
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