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

Understand

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.