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{sub tot} propor to r {sup -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 {sub e}sigma{sup 2} {sub e2}/2G (where R {sub e} is the half-light radius and sigma{sub e2} is the projected velocity dispersion), (3) strongly increase galaxy sizes (R {sub e} propor to M {sup 0.85+}-{sup 0.17}) and weakly increase velocity dispersions (sigma{sub e2} propor to M {sup 0.06+}-{sup 0.08}) with mass, thus moving galaxies away from the local observed M-R {sub e} and M-sigma{sub e2} relations, and (4) introduce substantial scatter in the M-R {sub e} and M-sigma{sub e2} relations. Our findings imply that-unless there is a high degree of fine tuning of themore » mix of progenitors and types of interactions-present-day massive early-type galaxies could not have assembled more than approx50% of their mass, and increased their size by more than a factor approx1.8, via dry merging.« less