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Compact dust emission in a gravitationally lensed massive quiescent galaxy at z = 2.15 revealed in ~130 pc-resolution observations by ALMA

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 gravitational lensing effect by the foreground cluster and the angular resolution provided by ALMA, our data reveal 1.2 mm continuum emission at $\sim130$ pc angular resolution. Compact dust continuum is detected at 7.9 $σ$ in the target but displaced from its stellar peak position by $62 \pm 38$ mas, or $\sim169 \pm 105$ pc in the source plane. We find considerably high dust-to-stellar mass ratio, $4 \times 10^{-4}$. From non-detection of the [C i] 3P2 -> 3P1 line, we then derive $3 σ$ upper limits on the molecular gas-to-dust mass ratio $δ_\mathrm{GDR} < 60$ and the molecular gas-to-stellar mass ratio fH2 < 2.3%. The derived $δ_\mathrm{GDR}$ is $>2\times$ smaller than the typical value assumed for quiescent galaxies in the literature. Our study supports that there exists a broad range of $δ_\mathrm{GDR}$ and urges submillimeter follow-up observations of quenching/recently quenched galaxies at similar redshifts. Based on the derived low $δ_\mathrm{GDR}$ and other observed dust 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.