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

TL;DR

This study analyzes how the central mass-density profiles of massive elliptical galaxies evolve over redshift, finding that profiles become steeper over time, likely due to dry mergers, with implications for galaxy evolution models.

Contribution

It provides the first evidence of significant evolution in the mass-density profiles of elliptical galaxies over 6 Gyr, accounting for selection biases and exploring alternative explanations.

Findings

Mass-density profiles become steeper at lower redshifts.

The evolution is not due to lensing selection biases.

Major dry mergers likely drive the observed profile changes.

Abstract

We present an analysis of the evolution of the central mass-density profile of massive elliptical galaxies from the SLACS and BELLS strong gravitational lens samples over the redshift interval z ~ 0.1-0.6, based on the combination of strong-lensing aperture mass and stellar velocity-dispersion constraints. We find a significant trend towards steeper mass profiles (parameterized by the power-law density model with rho ~ r^[-gamma]) at later cosmic times, with magnitude d<gamma>/dz = -0.60 +/- 0.15. We show that the combined lens-galaxy sample is consistent with a non-evolving distribution of stellar velocity dispersions. Considering possible additional dependence of <gamma> on lens-galaxy stellar mass, effective radius, and Sersic index, we find marginal evidence for shallower mass profiles at higher masses and larger sizes, but with a significance that is sub-dominant to the redshift dependence. Using the results of published Monte Carlo simulations of spectroscopic lens surveys, we verify that our mass-profile evolution result cannot be explained by lensing selection biases as a function of redshift. Interpreted as a true evolutionary signal, our result suggests that major dry mergers involving off-axis trajectories play a significant role in the evolution of the average mass-density structure of massive early-type galaxies over the past 6 Gyr. We also consider an alternative non-evolutionary hypothesis based on variations in the strong-lensing measurement aperture with redshift, which would imply the detection of an "inflection zone" marking the transition between the baryon-dominated and dark-matter halo-dominated regions of the lens galaxies. Further observations of the combined SLACS+BELLS sample can constrain this picture more precisely, and enable a more detailed investigation of the multivariate dependences of galaxy mass structure across cosmic time.