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HST Observations of
Gravitational Lensing and Cosmic Shear
R.E. Griфths et al.
Carnegie Mellon University, USA
(Co-authors:K.Ratnatunga/CMU,USA, S.Casertano/STScI,USA)
Space based observations of the gravitational lensing of faint
galaxies represent a new probe of cosmology. We report the strong
detection of galaxy-galaxy lensing in HST data taken with the
WFPC2 for the Medium Deep Survey. We also report the detec-
tion of cosmic shear, i.e. the weak gravitational lensing by dark
matter at moderate to high redshift. In the galaxy-galaxy lensing
study, we detect dark matter extending to about 50 half-light radii
around both elliptical and spiral galaxies. The result on cosmic
shear, at the level of about 2.8%, was measured using two inde-
pendent methods. This value is consistent with current models
for the lensing due to the large scale structure of dark matter.
We have also discovered ten new strong gravitational lens candi-
dates which require the resolution and depth of observations from
Space. i.e. faint systems with critical radii which are mostly sub-
arcsecond. We discuss the characteristics of the detected sample
to understand the selection eects and the discovery space. The
observed congurations have been described using simple mod-
els for the potential of the lensing galaxy. With the exception of
our rst lens discovery (Ratnatunga et al 1995 ApJ 453,L5), all
of these candidates require conrmation by spectroscopic observa-
tions which are diфcult from the ground at these faint magnitudes
and object separations.
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