Cold dust and low [O <scp>iii</scp>]/[C <scp>ii</scp>] ratios: an evolved star-forming population at redshift 7
Hiddo Algera, Hanae Inami, Laura Sommovigo, Yoshinobu Fudamoto, Raffaella Schneider, Luca Graziani, Pratika Dayal, R. J. Bouwens, Manuel Aravena, Elisabete da Cunha, Andrea Ferrara, A. P. S. Hygate, Ivana van Leeuwen, Ilse De Looze, Marco Palla, A. Pallottini, Renske Smit, Mauro Stefanon, Michael W. Topping, P. van der Werf
Abstract
ABSTRACT We present new ALMA Band 8 (rest-frame $90 \ \mu$m) continuum observations of three massive ($M_\star \approx 10^{10}\ \mathrm{ M}_\odot$) galaxies at z ≈ 7 previously detected in [C ii]$158 \ \mu$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 $T_\mathrm{dust}\approx 30-35\,$ 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 ($M_\mathrm{dust}\approx 10^{8}\ \mathrm{ M}_\odot$), albeit still consistent with models of high-redshift dust production. We furthermore target and detect [O iii]$88 \ \mu$m emission in both REBELS-12 and REBELS-25, and find $L_\mathrm{[O\, III]}/L_\mathrm{[C\, II]}\approx 1-1.5$ – low compared to the $L_\mathrm{[O\, \rm{\small III}]}/L_\mathrm{[C\, \rm{\small II}]}\gtrsim 2-10$ 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 $\mathrm{[O\, \rm{\small III}]}\lambda \lambda 4959,5007+\mathrm{H} \, \beta$ 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.