AAS 207th Meeting, 8-12 January 2006
Session 43 Galaxy Evolution
Oral, Monday, 10:00-11:30am, January 9, 2006, Balcony A

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[43.05] Bolometric Luminosities of Redshift Z~2 Galaxies as Inferred from Spitzer Observations

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\rm bol\ga3\times1011~L\odot.

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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.