Galaxy assembly

Summary

In a ΛCDM Universe, galaxies grow in mass both through star formation and through the addition of already-formed stars in galaxy mergers. Because of this partial decoupling of these two modes of galaxy growth, I discuss each separately in this biased and incomplete review of galaxy assembly—first giving an overview of the cosmic-averaged star formation history, and then moving on to discuss the importance of major mergers in shaping the properties of present-day massive galaxies. The cosmic-averaged star-formation rate, when integrated, is in reasonable agreement with the build-up of stellar mass density. Roughly 2/3 of all stellar mass is formed during an epoch of rapid star formation prior to z ∼ 1, with the remaining 1/3 formed in the subsequent 9 Gyr during a period of rapidly-declining star-formation rate. The epoch of important star formation in massive galaxies is essentially over. In contrast, a significant fraction of massive galaxies undergo a major merger at z ≲ 1, as evidenced by close-pair statistics, morphologically-disturbed galaxy counts, and the build-up of stellar mass in morphologically early-type galaxies. Each of these methods is highly uncertain; yet, taken together, it is not implausible that the massive galaxy population is strongly affected by late galaxy mergers, in excellent qualitative agreement with our understanding of galaxy evolution in a ΛCDM Universe.

Introduction

The last decade has witnessed amazing progress in our empirical and theoretical understanding of galaxy formation and evolution.