Measuring the Mass Dependence of Early-Type Galaxy Structure

Measuring the Mass Dependence of Early-Type Galaxy Structure We propose two-color ACS-WFC Snapshot observations of a sample of 118 candidate early-type gravitational lens galaxies. Our lens-candidate sample is selected to yield (in combination with earlier results) an approximately uniform final distribution of 40 early-type strong lenses across a wide range of masses, with velocity dispersions (a dynamical proxy for mass) ranging from 125 to 300 km/s. The proposed program will deliver the first significant sample of low-mass gravitational lenses. All of our candidates have known lens and source redshifts from Sloan Digital Sky Survey data, and all are bright enough to permit detailed photometric and stellar-dynamical observation. We will constrain the luminous and dark-matter mass profiles of confirmed lenses using lensed-image geometry and lens-galaxy structural/photometric measurements from HST imaging in combination with dynamical measurements from spatially resolved ground-based follow-up spectroscopy. Hence we will determine, in unprecedented detail, the dependence of early-type galaxy mass structure and mass-to-light ratio upon galaxy mass. These results will allow us to directly test theoretical predictions for halo concentration and star-formation efficiency as a function of mass and for the existence of a cuspy inner dark-matter component, and will illuminate the structural explanation behind the fundamental plane of early-type galaxies. The lens-candidate selection and confirmation strategy that we propose has been proven successful for high-mass galaxies by our Cycle 13 Snapshot program (10174). The program that we propose here will produce a complementary and unprecedented lens sample spanning a wide range of lens-galaxy masses.

We plan the same observing sequence for this program as was used in Cycle 13 program 10174: 1 x 420s ACS-WFC F435W 1 x 420s ACS-WFC F814W There should be one target per visit, one visit per target, and identical visit specifications for all. Targets are to be centered in the WFC1 aperture. No CR split, because we want to keep read noise down (we are read-noise limited at these short exposure times). 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