EARLY-TYPE HALO MASSES FROM GALAXY-GALAXY LENSING

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 CFHT using the UH8K CCD mosaic camera. Six 0.5 × 0.5 degree fields were observed for a total of 2 hours each in I and V, resulting in catalogs containing ~ 20000 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 < 77h^-1 kpc (z = 0.1) and 105 < r < 315h^-1 kpc (z = 0.9), and computed the mean tangential shear γ_T(θ) of the faint galaxies. The shear falls off with radius roughly as γ_T ∝ 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. Our results give a value for the average mean rotation velocity of an L_⋆ galaxy halo at r ~ 50 - 200h^-1 kpc of for a flat lambda (Ω_m0 = 0.3, Ω_λ0 = 0.7) cosmology ( for Einstein-de Sitter), and with little evidence for evolution with redshift. We find a mass-to-light ratio of M/L_B ≃ 121 ± 28h(r/100h^-1 kpc) (for L_⋆ galaxies) and these halos constitute Ω ≃ 0.04 ± 0.01(r/100h^-1 kpc) of closure density.