Significant Dust-obscured Star Formation in Luminous Lyman-break Galaxies at z ∼7-8

We make use of Atacama Large Millimeter/submillimeter Array continuum observations of 15 luminous Lyman-break galaxies at z ∼7-8 to probe their dust-obscured star formation. These observations are sensitive enough to probe obscured star formation rates (SFRs) of 20 M yr-1 (3σ). Six of the targeted galaxies show significant (≥3σ) dust-continuum detections, more than doubling the number of known dust-detected galaxies at z > 6.5. Their IR luminosities range from 2.7 × 1011 L to 1.1 × 1012 L, equivalent to obscured SFRs of 25 to 101 M yr-1. We use our results to quantify the correlation of the infrared excess (IRX) on the UV-continuum slope β UV and stellar mass. Our results are most consistent with a Small Magellanic Cloud (SMC) attenuation curve for intrinsic UV-slopes βUV,intr of -2.63 and most consistent with an attenuation curve in between SMC and Calzetti for βUV,intr slopes of -2.23, assuming a dust temperature T d of 50 K. Our fiducial IRX-stellar mass results at z ∼7-8 are consistent with marginal evolution from z ∼0. We then show how both results depend on T d . For our six dust-detected sources, we estimate their dust masses and find that they are consistent with dust production from supernovae if the dust destruction is low (<90%). Finally we determine the contribution of dust-obscured star formation to the SFR density for UV luminous (H<-21.5 mag: ≥ 1.7 L ∗UV) z ∼7-8 galaxies, finding that the total SFR density at z ∼7 and z ∼8 from bright galaxies is 0.20-0.10+0.10 dex and 0.23-0.09+0.06 dex higher, respectively; i.e., ∼1/3 of the star formation in ≥ 1.7 L ∗UV galaxies at z ∼7-8 is obscured by dust.