Program: GN-2012B-Q-78

How do massive early-type galaxies form? Merging is thought to be a key process, but the details are not clear: the important merger events could be minor or major, wet or dry. A key test for any theory of massive galaxy evolution is to explain the observed total density profiles of early-type galaxies, which can be precisely measured by joint analysis of strong gravitational lensing and stellar kinematics data. We are constructing a sample of 36 lenses at redshift 0.4--0.7, detected in the CFHTLS imaging survey, to investigate our tentative detection of an average steepening of the density profiles with cosmic time. To model each galaxy's profile slope accurately we need an accurate measurement of the lensed source redshift, which for some systems is best made in the NIR wavebands. With its broad wavelength coverage, GNIRS is very well-suited to this work: we propose to measure 9 source redshifts in this way in semester 2012B.