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

Abstract

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 ${\beta }_{\mathrm{UV},\mathrm{intr}}$ of -2.63 and most consistent with an attenuation curve in between SMC and Calzetti for ${\beta }_{\mathrm{UV},\mathrm{intr}}$ 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., ~ $\tfrac{1}{3}$ of the star formation in ≳1.7 L ^* _UV galaxies at z ~ 7-8 is obscured by dust.