The HDUV Survey: A Revised Assessment of the Relationship between UV Slope and Dust Attenuation for High-redshift Galaxies

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 M_(1600) = −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 × 10^9 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