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N. A. Reddy, C. C. Steidel (Caltech), D. Fadda, L. Yan (Spitzer Science Center), M. Pettini (IoA, Cambridge), A. E. Shapley (Princeton University), D. K. Erb (Center for Astrophysics), K. L. Adelberger (McKinsey & Co.)
We use Spitzer MIPS 24~\mum data to investigate
the bolometric luminosities of z~2 galaxies selected
by their optical (u, g r) and near-IR (b-k; j-k) colors, and
those selected by their submillimeter emission. We combine
our large sample of spectroscopic redshifts for u,g r
galaxies, which allow for unprecedented constraints on the
rest-frame mid-IR fluxes of z~2 galaxies, with
photometric redshifts for near-IR selected galaxies to
estimate rest-frame 5-8.5~\mum luminosities (L\rm
5-8.5~\mu m). Optical and near-IR selected galaxies have
L\rm 5-8.5\mu m that strongly correlate with their
infrared luminosities (L\rm IR) as determined from a
stacked X-ray analysis. We convert L\rm 5-8.5\mu m to
L\rm IR using local star-forming galaxies as templates,
and find that L\rm IR determined in this manner
reproduce those determined from a stacked X-ray analysis.
Our main results are as follows. Star-forming galaxies at
z~2 show a wide range in infrared luminosity from
~1010~L\odot to >1012~L\odot. The
average IR luminosity of optical/near-IR selected
star-forming galaxies indicates they are luminous infrared
galaxies (LIRGs) with \langle L\rm IR\rangle ~
3-5\times 1011~L\odot. MIPS observations allow for
an independent examination of dust extinction in high
redshift galaxies, and indicate that most star-forming
galaxies at z~2 follow the local relation between
rest-frame UV slope (\beta) and dust obscuration, implying
that \beta can be used to correct for extinction in these
galaxies. However, bright radio-detected submillimeter
galaxies (S\rm 850\mu m\ga 5~mJy) are at least 10
times more dust obscured than their UV spectral slopes would
indicate and, further, their submillimeter-inferred
bolometric luminosities are a factor of 2-10
systematically larger than those inferred from their
24~\mum fluxes. Young galaxies (\la 100~Myr) exhibit
less mid-IR flux for a given \beta than older galaxies, a
result that may reflect the timescale for hydrocarbon dust
production. . X-ray and 24~\mum stacks of u,g r galaxies
undetected at 24~\mum indicate their faintness is due
primarily to lower star formation rates (SFRs) and a
comparison between composite rest-frame UV spectra of
24~\mum detected and undetected galaxies shows the
latter are less dusty and/or have smaller dust covering
fractions. Comparison with local samples shows that galaxies
of a given bolometric luminosity are 2-10 times less
obscured by dust at z~2 than at the present epoch.
Such a trend in dust obscuration is expected as dust masses
of galaxies increase as they age and/or if the distribution
of dust in galaxies becomes more compact with time (e.g.,
via mergers), resulting in larger dust column densities
towards star-forming regions. We have examined L\rm bol
as a function of stellar mass and our results suggest that
\ga 70% of massive galaxies (M*\ga
1011~M\odot) in optical and near-IR surveys with
redshifts 1.5
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.