Dry Mergers: A Crucial Test for Galaxy Formation

Abstract

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 $\sim 6\times 10^{-5}$ Mpc${}^{-3}$ Gyr${}^{-1}$ at $z\le 1$ 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 to grow above a characteristic mass scale of $M_{\ast ,c}\sim 6.3\times 10^{10}$ M${}_{\odot }$ by mergers only. As a consequence of this transition, we find that around $M_{\ast ,c}$, 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 $10$ of galaxies more massive than $M_{\ast ,c}$ experience dry major mergers within their last Gyr at any given redshift $z\le 1$.

Replaced with MNRAS accepted version

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