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Published August 1, 1996 | Published
Journal Article Open

Weak Gravitational Lensing by Galaxies


We report a detection of weak, tangential distortion of the images of cosmologically distant, faint galaxies due to gravitational lensing by foreground galaxies. A mean image polarization of (p) = 0.011 ± 0.006 (95% confidence bounds) is obtained for 3202 pairs of source (23 < r_s ≤ 24) and lens (20 (20 ≤ r_d ≤ 23) galaxies with projected separations of 5" ≤ θ ≤ 34". Averaged over annuli of inner radius 5" and outer radius θ_max, the signal is string for lens-source separations of θ_max ≾ 90" consistent with quas-isothermal galaxy halos extending to large radii (≳100 h^(-1) kpc). The observed polarization is also consistent with the signal expected on the basis of simulations incorporating measured properties of local galaxies and modest extrapolations of the observed redshift distribution of faint galaxies (to which the results are somewhat sensitive). From the simulations we obtain formal best-fit model parameters for the dark halos of the lens galaxies that consist of a characteristic circular velocity of V* ~220 ± 80 km s^(-1) and characteristic radial extent of s* ≳ 100 h^(-1) kpc. The predicted polarization based on the model is relatively insensitive to the characteristic radial extent of the halos, s*, and very small halos (s* ~10 h^(-1) kpc) are excluded only at the 2 a level. The formal best-fit halo parameters imply typical masses for the lens galaxies within a radius of 100 h^-1 kpc on the order of 1.0_(-0.5)^(+1.2)^ x 10^(12) h^(-1) M⊙ (90% confidence bounds), in agreement with recent dynamical estimates of the masses of local spiral galaxies. This is particularly encouraging as the lensing and dynamical mass estimators rely on different sets of assumptions. Contamination of the gravitational lensing signal by a population of tidally distorted satellite galaxies can be ruled out with reasonable confidence. The prospects for corroborating and improving this measurement seem good, especially using deep HST archival data.

Additional Information

© 1996 The American Astronomical Society. Received 1995 March 20; accepted 1996 February 16. We are indebted to Jeremy Mould and Todd Small for acquiring the observations used for this analysis and to them, David Hogg, Gordon Squires, and Tony Tyson for helpful suggestions. Support under NSF contract AST 92-23370, the NASA HPCC program at Los Alamos National Laboratory (T. G. B.), and a NATO Advanced Fellowship (I. R. S.) is gratefully acknowledged.

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Published - 1996ApJ___466__623B.pdf


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