Properties of Galaxy Dark Matter Halos from Weak Lensing
Abstract
We present the results of a study of weak lensing by galaxies based on 45.5 deg^{2} of R_{C}band imaging data from the RedSequence 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 weaklensing 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 bestfit 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 NavarroFrenkWhite (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 bestfit velocity dispersion of σ=136+/5+/3 km s^{1} (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}_{28}h^{1} kpc for a galaxy with a fiducial luminosity of L_{B}=10^{10}h^{2}L_{B,solar} (under the assumption that the luminosity does not evolve with redshift). Alternatively, the bestfit NFW model yields a mass M_{200}=(8.4+/0.7+/0.4)×10^{11}h^{1}M_{solar} and a scale radius r_{s}=16.2^{+3.6}_{2.9}h^{1} kpc. This value for the scale radius is in excellent agreement with predictions from numerical simulations for a halo of this mass.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 2004
 DOI:
 10.1086/382726
 arXiv:
 arXiv:astroph/0306515
 Bibcode:
 2004ApJ...606...67H
 Keywords:

 Cosmology: Observations;
 Cosmology: Dark Matter;
 Galaxies: Halos;
 Cosmology: Gravitational Lensing;
 Astrophysics
 EPrint:
 Significantly revised version, accepted for publication in ApJ 11 pages, 6 figures