The effects and importance of galaxy merging in a cosmological context

Eric Bell

U Michigan

Mergers between dark matter halos, and the galaxies in them, are a central feature of Lambda CDM. In this talk, I explore the role of galaxy merging in setting the properties of galaxies, in particular the properties of non-star forming (quiescent) early-type galaxies. I will discuss a number of relevant considerations : the observation that the best structural predictor of quiescence is light profile shape (i.e., relative size of the bulge); the comparison between inferred merger rates and the rate of creation of early-type galaxies; the effect of merging on setting the scaling relations of present-day early-type galaxies, and the result that <~10% of cosmic-averaged star formation is directly triggered by mergers. I will argue that mergers are a crucial part of how early-type galaxies come into place. I will conclude with a number of potentially important open issues: the frequent existence of disks and/or rotation in early-type galaxies, controversies about the amount of evolution in stellar mass and size in the most massive early-type galaxies, the ever-open issue of the feedback mechanism that actually keeps cold gas out of galaxies, and thoughts about the importance of galaxy merging at small galaxy masses.