ADF22-WEB: Detection of a molecular gas reservoir in a massive quiescent galaxy located in a $z\approx 3$ proto-cluster core

Abstract

We present a study of the molecular gas reservoirs and dust contents in three quiescent galaxies (QGs) located in the core of the $z=3.09$ SSA22 proto-cluster. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we detect CO(3--2) emission in one galaxy, ADF22-QG1, marking the first direct detection of molecular gas in a quiescent galaxy from the early universe. The detected galaxy, ADF22-QG1, has a molecular gas mass of log$M_{{\mathrm{H}}_2}$/M${}_{\odot }=10.26\pm 0.07$ assuming a CO-to-H$2$ conversion factor ${\alpha }_{\mathrm{CO}}=4.4$ (log$M_{{\mathrm{H}}_2}$/M${}_{\odot }=9.52\pm 0.07$ for ${\alpha }_{\mathrm{CO}}=0.8$), corresponding to a gas mass fraction of $f_{\mathrm{gas}}\approx 14\%$ (2.5\%). The gas-to-dust ratio ${\delta }_{\mathrm{gdr}}\gtrsim 170$ (${\delta }_{\mathrm{gdr}}\gtrsim 30$) for ${\alpha }_{\mathrm{CO}}=4.4$ (${\alpha }_{\mathrm{CO}}=0.8$) is also derived for the first time for a QG at the epoch. For the other two galaxies, ADF22-QG2 and ADF22-QG3, non detections of CO(3--2) emission provide upper limits, $f_{\mathrm{gas}}\approx 17\%$ (3.1\%) and $f_{\mathrm{gas}}\approx 13\%$ (2.4\%), respectively. The inferred gas-consumption history of ADF22-QG1, based on its star-formation history, suggests that (i) dusty star-forming galaxies (DSFGs) at $z=4$--$6$ are plausible progenitors, and (ii) the cessation of gas accretion from cosmic web filaments plays an important role in their evolution to quenched systems. Furthermore, the presence of a detectable molecular gas reservoir in ADF22-QG1 indicates that additional mechanisms, such as morphological quenching, may be required to fully explain its quiescent nature.

8 pages, 4 figures, 1 table, submitted

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