Galaxy evolution from strong lensing statistics: the differential evolution of the velocity dispersion function in concord with the LambdaCDM paradigm

Abstract

We study galaxy evolution from z=1 to z=0 as a function of velocity dispersion sigma for galaxies with sigma > 95 km/s based on the measured and Monte Carlo realised local velocity dispersion functions (VDFs) of galaxies and the revised statistical properties of 30 strongly-lensed sources. We assume that the total (luminous plus dark) mass profile of a galaxy is isothermal in the optical region for 0 < z < 1 as suggested by mass modelling of lensing galaxies. For the evolutionary behaviours of the VDFs we find that: (1) the number density of massive (mostly early-type) galaxies with sigma > 200 km/s evolves differentially in the way that the number density evolution is greater at a higher velocity dispersion; (2) the number density of intermediate and low mass early-type galaxies (95 km/s < sigma < 200 km/s) is nearly constant; (3) the late-type VDF transformed from the Monte Carlo realised circular velocity function is consistent with no evolution in its shape or integrated number density consistent with galaxy survey results. These evolutionary behaviours of the VDFs are strikingly similar to those of the dark halo mass function (DMF) from N-body simulations and the stellar mass function (SMF) predicted by recent semi-analytic models of galaxy formation under the current LambdaCDM hierarchical structure formation paradigm. Interestingly, the VDF evolutions appear to be qualitatively different from ``stellar mass-downsizing'' evolutions obtained by many galaxy surveys. The coevolution of the DMF, the VDF and the SMF is investigated in quantitative detail in a following paper. We consider several possible systematic errors for the lensing analysis and find that they are not likely to alter the conclusions.(abridged)