Dust Obscuration and Metallicity at High Redshift: New Inferences from UV, Hα, and 8 μm Observations of z ~ 2 Star-forming Galaxies

Abstract

We use a sample of 90 spectroscopically confirmed Lyman break galaxies with Hα measurements and Spitzer MIPS 24 μm observations to constrain the relationship between rest-frame 8 μm luminosity (L_8) and star formation rate (SFR) for L* galaxies at z ~ 2. We find a tight correlation with 0.24 dex scatter between L_8 and Hα luminosity/SFR for z ~ 2 galaxies with 10^(10) L_☉ ≲ L_(IR) ≲ 10^(12) L_☉. Employing this relationship with a larger sample of 392 galaxies with spectroscopic redshifts, we find that the UV slope β can be used to recover the dust attenuation of the vast majority of moderately luminous L* galaxies at z ~ 2 to within a 0.4 dex scatter using the local correlation. Separately, young galaxies with ages ≲100 Myr appear to be less dusty than their UV slopes would imply based on the local trend and may follow an extinction curve that is steeper than what is typically assumed. Consequently, very young galaxies at high redshift may be significantly less dusty than inferred previously. Our results provide the first direct evidence, independent of the UV slope, for a correlation between UV and bolometric luminosity (L_(bol)) at high redshift, in the sense that UV-faint galaxies are on average less infrared and less bolometrically luminous than their UV-bright counterparts. The L_(bol)-L_(UV) relation indicates that as the SFR increases, L_(UV) turns over (or "saturates") around the value of L* at z ~ 2, implying that dust obscuration may be largely responsible for modulating the bright end of the UV luminosity function. Finally, dust attenuation is found to correlate with oxygen abundance at z ~ 2. Accounting for systematic differences in local and high-redshift metallicity calibrations, we find that L* galaxies at z ~ 2, while at least an order of magnitude more bolometrically luminous, exhibit ratios of metals to dust that are similar to those of local starbursts. This result is expected if high-redshift galaxies are forming their stars in a less metal-rich environment compared to local galaxies of the same luminosity, thus naturally leading to a redshift evolution in both the bolometric luminosity-metallicity and bolometric luminosity-obscuration relations.

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