130 research outputs found
Image Separation vs. Redshift of Lensed QSOs: Implications for Galaxy Mass Profiles
Recently, Park and Gott reported an interesting observation: image separation
of lensed QSOs declines with QSO redshift more precipitously than expected in
any realistic world model, if the lenses are taken to be either singular
isothermal spheres or point masses. In this Letter I propose that the observed
trend arises naturally if the lensing galaxies have logarithmic surface mass
density profiles that gradually change with radius. If the observed lack of
central (odd) images is also taken into account, the data favor a universal
dark matter density profile over an isothermal sphere with a core. Since the
trend of image separation vs. source redshift is mostly a reflection of galaxy
properties, it cannot be straightforwardly used as a test of cosmological
models. Furthermore, the current upper limits on the cosmological constant may
have to be revised.Comment: 6 pages, including 3 figures, LaTeX. Accepted to MNRA
Angular correlations between LBQS and APM: Weak Lensing by the Large Scale Structure
We detect a positive angular correlation between bright, high-redshift QSOs
and foreground galaxies. The QSOs are taken from the optically selected LBQS
Catalogue, while the galaxies are from the APM Survey. The correlation
amplitude is about a few percent on angular scales of over a degree. It is a
function of QSO redshift and apparent magnitude, in a way expected from weak
lensing, and inconsistent with QSO-galaxy correlations being caused by physical
associations, or uneven obscuration by Galactic dust. The correlations are
ascribed to the weak lensing effect of the foreground dark matter, which is
traced by the APM galaxies. The amplitude of the effect found here is compared
to the analytical predictions from the literature, and to the predictions of a
phenomenological model, which is based on the observed counts-in-cells
distribution of APM galaxies. While the latter agree reasonably well with the
analytical predictions (namely those of Dolag & Bartelmann 1997, and Sanz et
al. 1997), both under-predict the observed correlation amplitude on degree
angular scales. We consider the possible ways to reconcile these observations
with theory, and discuss the implications these observations have on some
aspects of extragalactic astronomy.Comment: 9 pages; MNRAS, in pres
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