Cold dust and low [O III]/[C II] ratios: an evolved star-forming population at redshift 7
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.
- Publication:
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Monthly Notices of the Royal Astronomical Society
- Pub Date:
- January 2024
- DOI:
- 10.1093/mnras/stad3111
- arXiv:
- arXiv:2301.09659
- Bibcode:
- 2024MNRAS.527.6867A
- Keywords:
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- galaxies: evolution;
- galaxies: high-redshift;
- submillimetre: galaxies;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 18 pages + appendices