Bolometric Luminosities of Redshift Z ˜ 2 Galaxies as Inferred from Spitzer Observations

Astronomy and Astrophysics – Astronomy

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We use Spitzer MIPS 24 μ m 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 μ m luminosities (L 5-8.5 μm). Optical and near-IR selected galaxies have L 5-8.5μm that strongly correlate with their infrared luminosities (L IR) as determined from a stacked X-ray analysis. We convert L 5-8.5μm to L IR using local star-forming galaxies as templates, and find that L 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&sun; to >1012 L&sun;. The average IR luminosity of optical/near-IR selected star-forming galaxies indicates they are luminous infrared galaxies (LIRGs) with < L IR> ˜ 3-5× 1011 L&sun;. 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 (β ) and dust obscuration, implying that β can be used to correct for extinction in these galaxies. However, bright radio-detected submillimeter galaxies (S 850μm⪆ 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 μ m fluxes. Young galaxies (⪉ 100 Myr) exhibit less mid-IR flux for a given β than older galaxies, a result that may reflect the timescale for hydrocarbon dust production. . X-ray and 24 μ m stacks of u,g r galaxies undetected at 24 μ m indicate their faintness is due primarily to lower star formation rates (SFRs) and a comparison between composite rest-frame UV spectra of 24 μ m 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 bol as a function of stellar mass and our results suggest that ⪆ 70% of massive galaxies (M*⪆ 1011 M&sun;) in optical and near-IR surveys with redshifts 1.5

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