Invited review to appear in "Planets to Cosmology: Essential Science in Hubble's Final Years", ed. M. Livio (Cambridge: CUP), in press (2005).
astro-ph/0408023

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GALAXY ASSEMBLY

Eric F. Bell


Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
bell@mpia.de


Abstract. In a LambdaCDM Universe, galaxies grow in mass both through star formation and through 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 ltapprox 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 LambdaCDM Universe.


Table of Contents

INTRODUCTION

THE ASSEMBLY OF STELLAR MASS
The cosmic-averaged star formation rate density
The cosmic-averaged star formation history
Are the cosmic star formation rates and histories consistent?

THE ASSEMBLY OF GALAXIES
The evolution of close galaxy pairs
The evolution of morphologically-disturbed galaxies
The evolution of plausible merger remnants
The importance of galaxy mergers since z ~ 1

SUMMARY AND OUTLOOK
Our increasing understanding of the local Universe
Using models as interpretive tools
The importance of multiple large HST fields

REFERENCES

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