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 ⊙ ≈10 10 M ⊙) galaxies at z ≈7 previously detected in [C II ]158 μm and underlying dust continuum emission in the Reionization Era Bright Emission Line Surv e y (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 T dust ≈30 -35 K in both of the dual-band detected targets, indicating they are colder than most known galaxies at z ∼7. Moreo v er, their inferred dust masses are large ( M dust ≈10 8 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. Ne vertheless, a lo w burstiness forms a natural explanation for the cold dust temperatures and low [O III ] λλ4959 , 5007 + H βequi v alent 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.