2022ApJ...938..144M

We present new observations of MRG-M2129, a quiescent galaxy at z = 2.15, with the Atacama Large Millimeter/submillimeter Array (ALMA). With the combination of the effect of gravitational lensing by the foreground galaxy cluster and the angular resolution provided by ALMA, our data reveal 1.2 mm continuum emission at ∼130 pc angular resolution. Compact dust continuum is detected at 7.9σ in the target but displaced from its stellar peak position by 62 ± 38 mas, or ∼169 ± 105 pc in the source plane. We find a considerably high dust-to-stellar mass ratio, 4 × 10^–4. From nondetection of the [C I] ³P₂ → ³P₁ line, we derive 3σ upper limits on the molecular gas-to-dust mass ratio δ_GDR < 60 and the molecular gas-to-stellar mass ratio f_H2 < 2.3%. The derived δ_GDR is ≳2× smaller than the typical value assumed for quiescent galaxies in the literature. Our study supports the idea that there exists a broad range of δ_GDR and urges submillimeter follow-up observations of quenching/recently quenched galaxies at similar redshifts. Based on the inferred low δ_GDR and other observed properties, we argue that the central black hole is still active and regulates star formation in the system. Our study exhibits a rare case of a gravitationally lensed type 2 QSO harbored by a quiescent galaxy.