2004ApJ...606...67H

We present the results of a study of weak lensing by galaxies based on 45.5 deg² of R_C-band imaging data from the Red-Sequence Cluster Survey (RCS). We define a sample of lenses with 19.5<R_C<21 and a sample of background galaxies with 21.5<R_C<24. We present the first weak-lensing detection of the flattening of galaxy dark matter halos. We use a simple model in which the ellipticity of the halo is f times the observed ellipticity of the lens. We find a best-fit value of f=0.77^+0.18_–0.21, which suggests that the dark matter halos are somewhat rounder than the light distribution. The fact that we detect a significant flattening implies that the halos are well aligned with the light distribution. Given the average ellipticity of the lenses, this implies a halo ellipticity of <e_halo≥0.33^+0.07_–0.09, in fair agreement with results from numerical simulations of cold dark matter. We note that this result is formally a lower limit to the flattening, since the measurements imply a larger flattening if the halos are not aligned with the light distribution. Alternative theories of gravity (without dark matter) predict an isotropic lensing signal, which is excluded with 99.5% confidence. Hence, our results provide strong support for the existence of dark matter. We also study the average mass profile around the lenses, using a maximum likelihood analysis. We consider two models for the halo mass profile: a truncated isothermal sphere (TIS) and a Navarro-Frenk-White (NFW) profile. We adopt observationally motivated scaling relations between the lens luminosity and the velocity dispersion and the extent of the halo. The TIS model yields a best-fit velocity dispersion of σ=136±5±3 km/s (all errors are 68% confidence limits; the first error bar indicates the statistical uncertainty, whereas the second error bar indicates the systematic error) and a truncation radius s=185⁺³⁰_–28h^–1 kpc for a galaxy with a fiducial luminosity of L_B=10¹⁰h^–2L_B,☉(under the assumption that the luminosity does not evolve with redshift). Alternatively, the best-fit NFW model yields a mass M₂₀₀=(8.4±0.7±0.4)x10¹¹h^–1M_☉and a scale radius r_s=16.2^+3.6_–2.9h^–1 kpc. This value for the scale radius is in excellent agreement with predictions from numerical simulations for a halo of this mass.