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ABSTRACT We present new ALMA Band 8 (rest-frame 90 \ m) continuum observations of three massive (M_ 10^10\ M_) galaxies at z ≈ 7 previously detected in C ii158 \ m and underlying dust continuum emission in the Reionization Era Bright Emission Line Survey (REBELS). We detect dust emission from two of our targets in Band 8 (REBELS-25 and REBELS-38), while REBELS-12 remains undetected. Through optically thin modified blackbody fitting, we determine dust temperatures of Tdust 30-35\, K in both of the dual-band detected targets, indicating they are colder than most known galaxies at z ∼ 7. Moreover, their inferred dust masses are large (Mdust 10^8\ M_), albeit still consistent with models of high-redshift dust production. We furthermore target and detect O iii88 \ m emission in both REBELS-12 and REBELS-25, and find LO\, III/LC\, II 1-1. 5 – low compared to the LO\, III/LC\, II 2-10 observed in the known z ≳ 6 population thus far. We argue the lower line ratios are due to a comparatively weaker ionizing radiation field resulting from the less starbursty nature of our targets, although the possibility of REBELS-12 being a merger of an O iii-bright and O iii-faint component prevents the unambiguous interpretation of its O iii/C ii ratio. Nevertheless, a low burstiness forms a natural explanation for the cold dust temperatures and low O\, III 4959, 5007+H \, equivalent widths of REBELS-25 and REBELS-38. Overall, these observations provide evidence for the existence of a massive, dust-rich galaxy population at z ≈ 7 which has previously experienced vigorous star formation, but is currently forming stars in a steady, as opposed to bursty, manner.
Algera et al. (Wed,) studied this question.