Galaxy Halo Masses from Galaxy‐Galaxy Lensing

Abstract

We present measurements of the extended dark halo profiles of bright early-type galaxies at redshifts 0.1 < z < 0.9 obtained via galaxy-galaxy lensing analysis of images taken at the Canada-France-Hawaii Telescope using the UH8K CCD mosaic camera. Six 05 × 05 fields were observed for a total of 2 hr each in I and V, resulting in catalogs containing ~20,000 galaxies per field. We used V-I color and I magnitude to select bright early-type galaxies as the lens galaxies, yielding a sample of massive lenses with fairly well-determined redshifts and absolute magnitudes M ~ M* ± 1. We paired these with faint galaxies lying at angular distances 20'' < θ < 60'', corresponding to physical radii of 26 < r < 77 h-1 kpc (z = 0.1) and 105 < r < 315 h-1 kpc (z = 0.9), and computed the mean tangential shear γT(θ) of the faint galaxies. The shear falls off with radius roughly as γT ∝ 1/θ as expected for flat rotation curve halos. The shear values were weighted in proportion to the square root of the luminosity of the lens galaxy. This is optimal if the halo mass at a given radius varies as M ∝ , as is the case at smaller radii, and in this context our results give a value for the average mean rotation velocity of an L* galaxy halo at r ~ 50-200 h-1 kpc of v* = 238 km s-1 for a flat lambda (Ωm0 = 0.3,Ωλ0 = 0.7) cosmology (v* = 269 km s-1 for Einstein-de Sitter) and with little evidence for evolution with redshift. These halo masses are somewhat (2-3 times) lower than a simple perfectly flat rotation curve extrapolation from smaller scale dynamical measurements. They are also considerably lower than the masses of halos found from the best-studied X-ray halos. They do, however, agree extremely well with the masses of halos of the same abundance in lambda-CDM simulations. We find a mass-to-light ratio of M/LB ≃ 121 ± 28h(r/100 h-1 kpc) (for L* galaxies) and these halos constitute Ω ≃ 0.04 ± 0.01(r/100 h-1 kpc) of closure density.