Dry Mergers: A Crucial Test for Galaxy Formation

TL;DR

This paper examines the significance of dry mergers in the formation of massive galaxies within the cold dark matter framework, highlighting a transition mass scale and its effects on galaxy growth and merger characteristics.

Contribution

It introduces an empirical star formation shut-off mass scale and analyzes its impact on dry merger rates and galaxy evolution, providing new insights into galaxy assembly processes.

Findings

Dry merger rate remains nearly constant at low redshift.

Less than half of dry mergers occur between two early-type galaxies.

A transition mass scale influences the prevalence of equal-mass mergers.

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 $z \leq 1$ 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 $M_{*,c} \sim 6.3 \times 10^{10}$ M$_{\odot}$ by mergers only. 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 any given redshift $z \le 1$.