2018ApJ...853...56R

We use a newly assembled sample of 3545 star-forming galaxies with secure spectroscopic, grism, and photometric redshifts at z = 1.5-2.5 to constrain the relationship between UV slope (β) and dust attenuation (L_IR/L_UV ≡ IRX). Our sample significantly extends the range of L_UV and β probed in previous UV-selected samples, including those as faint as M₁₆₀₀ = -17.4 (≃0.05L_UV^* ) and -2.6 <= β <= 0.0. IRX is measured using stacks of deep Herschel data, and the results are compared with predictions of the IRX-β relation for different assumptions of the stellar population model and obscuration curve. We find that z = 1.5-2.5 galaxies have an IRX-β relation that is consistent with the predictions for an SMC curve if we invoke subsolar-metallicity models currently favored for high-redshift galaxies, while the commonly assumed starburst curve overpredicts the IRX at a given β by a factor of >=3. IRX is roughly constant with L_UV for L_UV >= 3 x 10⁹ L_☉. Thus, the commonly observed trend of fainter galaxies having bluer β may simply reflect bluer intrinsic slopes for such galaxies, rather than lower obscurations. The IRX-β relation for young/low-mass galaxies at z >= 2 implies a dust curve that is steeper than the SMC. The lower attenuations and higher ionizing photon output for low-metallicity stellar populations point to Lyman continuum production efficiencies, ξ_ion, that may be elevated by a factor of ≃2 relative to the canonical value for L* galaxies, aiding in their ability to keep the universe ionized at z ∼ 2.