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
For a PDF version of the article, click
here.
GALAXY ASSEMBLY
Eric F. Bell
Max-Planck-Institut
für Astronomie,
Königstuhl 17, D-69117 Heidelberg, Germany
bell@mpia.de
Abstract.
In a 
CDM 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 
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
CDM Universe.
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