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 β UV , intr of −2.63 and most consistent with an attenuation curve in between SMC and Calzetti for β UV , 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.

Reionization Era Bright Emission Line Survey: Selection and Characterization of Luminous Interstellar Medium Reservoirs in the z > 6.5 Universe

“…Figure 11 (top panel) shows how the sources with 7σ ISM-cooling lines are distributed as a function of redshift and UV luminosity. Also shown in the top panel of Figure 11 are sources from the two pilot programs to REBELS (Smit et al 2018;Schouws et al 2022aSchouws et al , 2022b.…”

“…This paper is based on data obtained with the ALMA Observatory, under the Large Program 2019.1.01634.L. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with (Bowler et al 2018;Schouws et al 2022aSchouws et al , 2022b.…”

“…The remaining observations appear likely to be executed in March 2022 when the configuration of ALMA is in C43-1 or C43-2, according to the current JAO schedule. (Smit et al 2018;Schouws et al 2022aSchouws et al , 2022b and the literature (Hashimoto et al 2019;Tamura et al 2019;Bakx et al 2020;Harikane et al 2020). The dashed lines show the break-even ratios assuming three separate tunings for a spectral scan.…”

“…Fortunately, thanks to the improving capabilities of the Atacama Large Millimeter Array (ALMA), we can make significant progress characterizing the physical properties of massive star-forming sources in the early universe (e.g., Hodge & da Cunha 2020). ALMA efficiently allows for a probe of the redshift and dynamical state of galaxies in the reionization epoch using bright interstellar medium (ISM)-cooling lines like 157.74 μm [C II] and 88.36 μm [O III] (e.g., Smit et al 2018;Hashimoto et al 2019;Tamura et al 2019;Jones et al 2021), while simultaneously probing the far-IR dust-continuum radiation (e.g., Capak et al 2015;Bowler et al 2018;Fudamoto et al 2020;Schouws et al 2022a). This provides us with an essential measure of the star formation in galaxies obscured by dust.…”

“…The effectiveness of ALMA in probing the physical properties of galaxies has been demonstrated by a few particularly noteworthy observational programs with ALMA, e.g., the ASPECS ALMA Large Program in cycle 4 probing the dust and gas in z > 0.5 galaxies over the Hubble Ultra Deep Field (e.g., Aravena et al 2020;Bouwens et al 2020;Decarli et al 2020;González-López et al 2020;Walter et al 2020) and the ALPINE ALMA Large Program in cycle 5 targeting >110 z = 4-6 sources (Béthermin et al 2020;Faisst et al 2020;Le Fevre et al 2020). By contrast, the number of sources in the reionization epoch that have been characterized in detail with ALMA has been modest (e.g., Maiolino et al 2015;Watson et al 2015;Inoue et al 2016;Pentericci et al 2016;Knudsen et al 2017;Laporte et al 2017Laporte et al , 2019Matthee et al 2017Matthee et al , 2019Hashimoto et al 2018Hashimoto et al , 2019Smit et al 2018;Tamura et al 2019;Bakx et al 2020;Schouws et al 2022a).…”