Cold Dust and Low [OIII]/[CII] Ratios: an Evolved Star-forming Population at Redshift 7

Abstract

We present new ALMA Band 8 (rest-frame $90\hat{I}¼$m) observations of three massive ($M_{\star }\approx 10^{10}{M}_{\odot }$) galaxies at $z\approx 7$ previously detected in [CII]$158\hat{I}¼$m and underlying dust continuum emission in the Reionization Era Bright Emission Line Survey (REBELS). We detect the dust continuum emission of two of our targets in Band 8 (REBELS-25 and REBELS-38), while REBELS-12 remains undetected. Through modified blackbody fitting we determine cold dust temperatures ($T_{\mathrm{dust}}\approx 30-35$K) in both of the dual-band detected targets, given a fiducial model of optically thin emission with ${\hat{I}}^2=2.0$. Their dust temperatures are lower than most $z\sim 7$ galaxies in the literature, and consequently their dust masses are higher ($M_{\mathrm{dust}}\approx 10^8{M}_{\odot }$). Nevertheless, these large dust masses are still consistent with predictions from models of dust production in the early Universe. In addition, we target and detect [OIII]$88\hat{I}¼$m emission in both REBELS-12 and REBELS-25, and find $L_{[\mathrm{OIII}]}/L_{[\mathrm{CII}]}$ ratios of approximately unity, low compared to the $L_{[\mathrm{OIII}]}/L_{[\mathrm{CII}]}\gtrsim 2-10$ observed in the known $z\gtrsim 6$ 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. This low burstiness supports the cold dust temperatures and below average $[\mathrm{OIII}]\hat{I}»\lambda 4959,5007+\mathrm{H}{\hat{I}}^2$ equivalent widths of REBELS-25 and REBELS-38, compared to the known high-redshift population. Overall, this provides evidence for the existence of a massive, dust-rich galaxy population at $z\approx 7$ which has previously experienced vigorous star formation, but is currently forming stars in a steady, as opposed to bursty, manner.

18 pages + appendices; 8 figures in main text; Submitted to MNRAS on 27 Dec 2022