Several studies have suggested that the observed size evolution of massive early-type galaxies (ETGs) can be explained as a combination of dry mergers and progenitor bias, at least since z = 1. In this paper we carry out a new test of the dry-merger scenario based on recent lensing measurements of the evolution of the mass density profile of ETGs. We construct a theoretical model for the joint evolution of the size and density slope \gamma’ driven by dry mergers occurring at rates given by cosmological simulations. We show that a dry-merger scenario cannot reproduce the almost constant \gamma’ in the range 0 < z < 1, being ruled out at > 99% CL. We then show with a simple toy model that a modest amount of cold gas in the mergers – consistent with the upper limits on recent star formation in ETGs – is sufficient to reconcile the model with the observations. We thus suggest a scenario where the outer regions of massive ETGs grow by dry-merger accretion while small amounts of dissipation and nuclear star formation conspire to keep the mass density profile constant and approximately isothermal.
Date added: Tue, 15 Oct 13