Massive Dark-matter halos and Evolution of Early-type Galaxies to z=1

Abstract

(Shortened) The combination of lensing and stellar dynamics breaks the mass-anisotropy degeneracy and provides stringent constraints on the mass distribution in early-type (E/S0) galaxies out to z~1. We present the combined results from the five field E/S0 lens galaxies at z=0.5-1.0 analyzed as part of the LSD Survey. We find: (i) Constant M/L models are ruled out at >99% CL for all five E/S0s. The projected dark-matter mass fractions inside the Einstein (effective) radius is f_DM=0.37-0.72 (0.15-0.65) for isotropic models. (ii) The average power-law slope of the total mass distribution is <\gamma'>=1.75+-0.10 for isotropic models with 0.20 rms scatter. The ratio between the observed central stellar velocity dispersion and that from the best-fit SIE lens model is <f_SIE>=<\sigma/\sigma_SIE>=0.87+-0.04 with 0.08 rms. Considering that \gamma'>2 and f_SIE>1 have been reported for other systems, we conclude that there is a significant intrinsic scatter in the density slopes of E/S0s (rms \~15%). Hence, the isothermal approximation is not sufficiently accurate for applications that depend critically on the slope of the mass density profile (i.e. measuring H_0). (iii) The inner power-law slope of the dark-matter halo is constrained to be <\gamma>=1.3(+0.2/-0.4) (68% CL) for the isotropic model or an upper limit of \gamma<0.6, if the galaxies are radially anisotropic (r_i=R_e). This is consistent with numerical simulations only for an isotropic velocity ellipsoid and if baryonic collapse and star-formation do not steepen dark-matter density profiles. (iv) The average stellar M/L evolves as d\log(M_*/L_B)/dz =-0.72+-0.10, obtained via the FP. Based on lensing and dynamics we find d\log(M_*/L_B)/dz=-0.75+-0.17, indicating that the M/L ratio evolution for our sample of field E/S0s is faster than those in clusters.