Compact Dust Emission in a Gravitationally Lensed Massive Quiescent Galaxy at <i>z</i>=2.15 Revealed in ∼130 pc Resolution Observations by the Atacama Large Millimeter/submillimeter Array

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 similar to 130 pc angular resolution. Compact dust continuum is detected at 7.9 sigma in the target but displaced from its stellar peak position by 62 +/- 38 mas, or similar to 169 +/- 105 pc in the source plane. We find a considerably high dust-to-stellar mass ratio, 4 x 10(-4). From nondetection of the [C i] P-3(2) -> P-3(1) line, we derive 3 sigma upper limits on the molecular gas-to-dust mass ratio delta (GDR) < 60 and the molecular gas-to-stellar mass ratio f (H2) < 2.3%. The derived delta (GDR) is greater than or similar to 2x smaller than the typical value assumed for quiescent galaxies in the literature. Our study supports the idea that there exists a broad range of delta (GDR) and urges submillimeter follow-up observations of quenching/recently quenched galaxies at similar redshifts. Based on the inferred low delta (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.