DRY MERGERS AND THE FORMATION OF EARLY-TYPE GALAXIES: CONSTRAINTS FROM LENSING AND DYNAMICS

Dissipationless (gas-free or dry) mergers have been suggested to play a major role in the formation and evolution of early-type galaxies, particularly in growing their mass and size without altering their stellar populations. We perform a new test of the dry-merger hypothesis by comparing N-body simulations of realistic systems to empirical constraints provided by recent studies of lens early-type galaxies. We find that major and minor dry mergers (1) preserve the nearly isothermal structure (rho(tot) proportional to r(-2)) of early-type galaxies within the observed scatter, (2) do not change more than the observed scatter the ratio between total mass M and virial mass R(e)sigma(2)(e2)/2G (where R(e) is the half-light radius and sigma(e2) is the projected velocity dispersion), (3) strongly increase galaxy sizes (R(e) proportional to M(0.85 +/- 0.17)) and weakly increase velocity dispersions (sigma(e2) proportional to M(0.06 +/-) (0.08)) with mass, thus moving galaxies away from the local observed M-R(e) and M-sigma(e2) relations, and (4) introduce substantial scatter in the M-R(e) and M-sigma(e2) relations. Our findings imply that-unless there is a high degree of fine tuning of the mix of progenitors and types of interactions-present-day massive early-type galaxies could not have assembled more than similar to 50% of their mass, and increased their size by more than a factor similar to 1.8, via dry merging.