A Multi-Wavelength Census of Star Formation at Redshift z~2

We examine the census of star-forming galaxies and their extinction
properties at redshift z~2, when a large fraction of the stellar mass
in the universe formed. We find a good agreement between the X-ray,
radio, and de-reddened UV estimates of the average star formation rate
(SFR) for our sample of z~2 galaxies of ~50 Msun/yr, indicating that
the locally calibrated SFR relations appear to be statistically valid
from redshifts 1.5 extinction properties of individual star-forming galaxies, and we find
that the rest-frame UV slope of most galaxies at z~2 can be used to
infer their attenuation factors, L(bol)/L(UV). As in the local
universe, the obscuration, L(FIR)/L(UV), is strongly dependent on
bolometric luminosity, and ranges in value from <1 to ~1000 within the
sample considered. However, the obscuration is ~10 times smaller at a
given L(bol) (or, equivalently, a similar level of obscuration occurs
at luminosities ~10 times larger) at z~2 than at z=0. This trend is
expected as galaxies age and their gas becomes enriched with dust.
Specific SFRs indicate a wide range in the evolutionary state of
galaxies at z~2, from galaxies that have just begun to form stars to
those that have already accumulated most of their stellar mass and are
about to become, or already are, passively evolving. Finally, we
examine two techniques for assessing the census of galaxies at z~2.
In the first, we select galaxies using optical, near-IR, and sub-mm
criteria, and find a large overlap between optical and near-IR
selected samples of galaxies at z~2. The second technique involves
reconstructing the luminosity function of z~2 galaxies as determined
from Monte Carlo simulations. We find that the SFR density increases
between z=4 to z=2 by a factor of ~1.2. However, this increase is
mild compared to the much steeper increase from z~6 to z~4, suggesting
that the processes responsible for the increase in SFR density at
early epochs saturate around z~2.