Dry Mergers: A Crucial Test for Galaxy Formation

We investigate the role that dry mergers play in the build-up of massive galaxies within the cold dark matter paradigm. Implementing an empirical shut-off mass scale for star formation, we find a nearly constant dry merger rate of ∼ 6× 10 Mpc Gyr at z 6 1 and a steep decline at larger z. Less than half of these mergers are between two galaxies that are morphologically classified as early-types, and the other half is mostly between an early-type and late-type galaxy. Latter are prime candidates for the origin of tidal features around red elliptical galaxies. The introduction of a transition mass scale for star formation has a strong impact on the evolution of galaxies, allowing them only to grow by mergers above a characteristic mass scale of M∗,c ∼ 6.3× 10 10 M⊙. As a consequence of this transition, we find that around M∗,c, the fraction of 1:1 mergers is enhanced with respect to unequal mass major mergers. This suggest that it is possible to detect the existence of a transition mass scale by measuring the relative contribution of equal mass mergers to unequal mass mergers as a function of galaxy mass. The evolution of the high-mass end of the luminosity function is mainly driven by dry mergers at low z. We however find that only 10% − 20% of galaxies more massive than M∗,c experience dry major mergers within their last Gyr at z 6 1, and conclude that dry mergers do not significantly change the mass function at the high-mass end.