The merger rate of galaxies in the Illustris Simulation: a comparison with observations and semi-empirical models

Understand

We have constructed merger trees for galaxies in the Illustris Simulation by directly tracking the baryonic content of subhalos.

• These merger trees are used to calculate the galaxy-galaxy merger rate as a function of descendant stellar mass, progenitor stellar mass ratio, and redshift.
• We demonstrate that the most appropriate definition for the mass ratio of a galaxy-galaxy merger consists in taking both progenitor masses at the time when the secondary progenitor reaches its maximum stellar mass.
• Additionally, we avoid effects from `orphaned' galaxies by allowing some objects to `skip' a snapshot when finding a descendant, and by only considering mergers which show a well-defined `infall' moment.