Properties of galaxy dark matter halos from weak lensing

We present the results of a study of the average mass profile around galaxies using weak gravitational lensing. We use 45.5 deg^2 of R_C band imaging data from the Red-Sequence Cluster Survey (RCS) and define a sample of 1.2\times 10^5 lenses with 19.5<R_C<21, and a sample of 1.5\times 10^6 background galaxies with 21.5<R<24. We constrain the power law scaling relations between the $B$-band luminosity and the mass and size of the halo, and find that the results are in excellent agreement with observed luminosity-line-width relations. Under the assumption that the luminosity does not evolve with redshift, the best fit NFW model yields a mass M_{200}=(8.8\pm0.7)\times 10^{11} h^{-1} Msun and a scale radius r_s=16.7^{+3.7}_{-3.0} h^{-1} kpc for a galaxy with a fiducial luminosity of \lumstar. The latter result is in excellent agreement with predictions from numerical simulations for a halo of this mass. We also observe a signficant anisotropy of the lensing signal around the lenses, implying that the halos are flattened and aligned with the light distribution. We find an average projected) halo ellipticity of <e_{\rm halo}>=0.20^{+0.04}_{-0.05}, in fair agreement with results from numerical simulations of CDM. 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.