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 : i ) preserve the nearly isothermal structure ( \rho _ { tot } \propto r ^ { -2 } ) of early-type galaxies within the observed scatter ; ii ) do not change more than the observed scatter the ratio between total mass M and “ virial ” mass R _ { e } \sigma _ { e 2 } ^ { 2 } / 2 G ( where R _ { e } is the half-light radius and \sigma _ { e 2 } the projected velocity dispersion ) ; iii ) increase strongly galaxy sizes ( R _ { e } \propto M ^ { 0.85 \pm 0.17 } ) and weakly velocity dispersions ( \sigma _ { e 2 } \propto M ^ { 0.06 \pm 0.08 } ) with mass , thus moving galaxies away from the local observed M - R _ { e } and M - \sigma _ { e 2 } relations ; iv ) introduce substantial scatter in the M - R _ { e } and M - \sigma _ { e 2 } 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 can not have assembled more than \sim 50 \% of their mass , and increased their size by more than a factor \sim 1.8 , via dry merging .