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

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We make use of ALMA continuum observations of 15 luminous Lyman-break galaxies at z78 to probe their dust-obscured star-formation. These observations are sensitive enough to probe to obscured SFRs of 20 M/yr (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 20 to 105 M/yr. We use our results to quantify the correlation of the infrared excess IRX on the UV-continuum slope βUV and stellar mass. Our results are most consistent with an 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 Td of 50 K. Our fiducial IRX-stellar mass results at z78 are consistent with marginal evolution from z0. We then show how both results depend on Td. For our six dust-detected sources, we estimate their dust masses and find that they are consistent with dust production from SNe if the dust destruction is low (<90%). Finally we determine the contribution of dust-obscured star formation to the star formation rate density for UV luminous (<21.5 mag: 1.7LUV) z78 galaxies, finding that the total SFR density at z7 and z8 from bright galaxies is 0.18−0.10+0.08 dex and 0.20−0.09+0.05 dex higher, respectively, i.e. 13 of the star formation in 1.7LUV galaxies at z78 is obscured by dust.

astro-ph.GA, astro-ph.GA
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Astrophysical Journal
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American Astronomical Society
Science and Technology Facilities Council (ST/M001172/1)
European Research Council (695671)
STFC (ST/V000918/1)