The BOSS Emission-Line Lens Survey. II. Investigating Mass-density Profile Evolution in the SLACS+BELLS Strong Gravitational Lens Sample

We present an analysis of the evolution of the central mass-densityprofile of massive elliptical galaxies from the SLACS and BELLS stronggravitational lens samples over the redshift interval z ≈ 0.1-0.6,based on the combination of strong-lensing aperture mass and stellarvelocity-dispersion constraints. We find a significant trend towardsteeper mass profiles (parameterized by the power-law density model withρvpropr ^-γ) at later cosmic times, with magnitude d γ > /dz = -0.60 ± 0.15. We show that the combinedlens-galaxy sample is consistent with a non-evolving distribution ofstellar velocity dispersions. Considering possible additional dependenceof γ > on lens-galaxy stellar mass, effective radius, andSérsic index, we find marginal evidence for shallower massprofiles at higher masses and larger sizes, but with a significance thatis subdominant to the redshift dependence. Using the results ofpublished Monte Carlo simulations of spectroscopic lens surveys, weverify that our mass-profile evolution result cannot be explained bylensing selection biases as a function of redshift. Interpreted as atrue evolutionary signal, our result suggests that major dry mergersinvolving off-axis trajectories play a significant role in the evolutionof the average mass-density structure of massive early-type galaxiesover the past 6 Gyr. We also consider an alternative non-evolutionaryhypothesis based on variations in the strong-lensing measurementaperture with redshift, which would imply the detection of an"inflection zone" marking the transition between the baryon-dominatedand dark-matter halo-dominated regions of the lens galaxies. Furtherobservations of the combined SLACS+BELLS sample can constrain thispicture more precisely, and enable a more detailed investigation of themultivariate dependences of galaxy mass structure across cosmic time.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprograms 10174, 10494, 10587, 10798, 10886, and 12209.