Session 13 - Gravitational Effects.
Display session, Monday, June 10
Great Hall,

## [13.02] Weak Gravitational Lensing by Galaxies - Implications for Dark Matter Halos

T. G. Brainerd (Boston U.), R. D. Blandford (Caltech), I. Smail (U. Durham)

A detection of weak, tangential distortion of the images of cosmologically distant, faint galaxies caused by gravitational lensing by foreground galaxies is reported. The imaging data consist of a 24 ksec integration in Gunn-r of a single blank field obtained with the COSMIC imaging spectrograph on the Hale 5-m telescope. The orientations of faint (23 < r \le 24) galaxies relative to brighter (20 \le r \le 23) galaxies is investigated. The distribution of the orientations of the faint galaxy images is inconsistent with a uniform distribution at a confidence level of 99.9%. Averaged over annuli of inner radius 5" and outer radius 34" (centered on the bright galaxies), an image polarization of \left< p \right> = 0.011 \pm 0.006 (95% confidence bounds) is found for the faint galaxies. The variation of \left< p \right> with both differential lens-source separation and the maximum radius of the annulus is investigated.

>From Monte Carlo simulations of galaxy-galaxy lensing that incorporate measured properties of local galaxies and modest extrapolations of the observed redshift distribution of faint galaxies, formal best-fit parameters for the dark matter halos associated with the lens galaxies are obtained. For L^\ast galaxies a characteristic circular velocity of V^\ast \sim 220\pm 80 km/s and outer scale radius of s^\ast \ge 100h^-1 kpc is found. These parameters imply a typical mass for the lens galaxies within 100 h^-1 kpc of order 1.0^+1.2_-0.5 \times 10^12 h^-1 M_ødot (90% confidence bounds), in good agreement with recent dynamical estimates of the masses of local spiral galaxies