Invisible Major Mergers: Why the Definition of a Galaxy Merger Ratio Matters

Abstract

The mapping between dark matter halo mass, galaxy stellar mass, and galaxy cold gas mass is not a simple linear relation, but is influenced by a wide array of galaxy formation processes. We implement observationally-normalized relations between dark matter halo mass, stellar mass, and cold gas mass to explore these mappings, with specific emphasis on the correlation between different definitions of a major galaxy merger. We always define a major merger by a mass ratio m/M>0.3, but allow the masses used to compute this ratio to be defined in one of three ways: dark matter halo masses, galaxy stellar masses, or galaxy baryonic masses (stars and cold gas). We find that the merger ratio assigned to any particular merger event depends strongly on which of these masses is used, with the mapping between different mass ratio definitions showing strong evolution with halo mass and redshift. For example, major dark matter mergers (>0.3) in small galaxies (M_DM < 10^11 Msun) typically correspond to very minor stellar mergers (<1/20). These mergers contain significant dark matter mass, and should cause noticable morphological disruption to the primary galaxy, even though there is no observable bright companion. In massive galaxies, there is an opposite effect, with bright companion galaxies corresponding to only minor dark matter mergers. We emphasize that great care must be taken when comparing mergers based on different mass ratio definitions.