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]λλ4959, 5007 + Hβ 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.