Cold dust and low [O iii]/[C ii] ratios: an evolved star-forming population at redshift 7

We present new ALMA Band 8 (rest-frame 90 \ μm) continuum observations of three massive (M_ ≈ 10¹⁰\ M_) galaxies at z ≈ 7 previously detected in [C ii]158 \ μm 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≈ 10⁸\ M_), albeit still consistent with models of high-redshift dust production. We furthermore target and detect [O iii]88 \ μm emission in both REBELS-12 and REBELS-25, and find L_[O\, III]/L_[C\, II]≈ 1-1.5 – low compared to the L[O\, III]/L[C\, 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]λ λ 4959,5007+H \, β 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.