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Several studies have suggested that the observed size evolution of massiveearly-type galaxies (ETGs) can be explained as a combination of dry mergers andprogenitor bias, at least since z\sim1. In this paper we carry out a new testof the dry-merger scenario based on recent lensing measurements of theevolution of the mass density profile of ETGs. We construct a theoretical modelfor the joint evolution of the size and mass density profile slope \gamma'driven by dry mergers occurring at rates given by cosmological simulations.Such dry-merger model predicts a strong decrease of \gamma' with cosmic time,inconsistent with the almost constant \gamma' inferred from observations in theredshift range 0<z<1. We then show with a simple toy model that a modest amountof cold gas in the mergers -- consistent with the upper limits on recent starformation in ETGs -- is sufficient to reconcile the model with measurements of\gamma'. By fitting for the amount of gas accreted during mergers, we find thatmodels with dissipation are consistent with observations of the evolution inboth size and density slope, if \sim4% of the total final stellar mass arisesfrom the gas accreted since z\sim1. Purely dry merger models are ruled out at >99% CL. We thus suggest a scenario where the outer regions of massive ETGs growby accretion of stars and dark matter, while small amounts of dissipation andnuclear star formation conspire to keep the mass density profile constant andapproximately isothermal.

Purely dry mergers do not explain the observed evolution of massive early-type galaxies since z = 1