2023ApJ...958..186C

We present 0.″22-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO(2-1) emission from the circumnuclear gas disk in the red nugget relic galaxy PGC 11179. The disk shows regular rotation, with projected velocities near the center of 400 km s^–1. We assume the CO emission originates from a dynamically cold, thin disk and fit gas-dynamical models directly to the ALMA data. In addition, we explore systematic uncertainties by testing the impacts of various model assumptions on our results. The supermassive black hole (BH) mass (M_BH) is measured to be M_BH = (1.91 ± 0.04 [1σ statistical] ${}_{-0.51}^{+0.11}$ [systematic]) × 10⁹M_⊙, and the H-band stellar mass-to-light ratio M/L_H = 1.620 ± 0.004 [1σ statistical] ${}_{-0.107}^{+0.211}$ [systematic] M_⊙/L_⊙. This M_BH is consistent with the BH mass-stellar velocity dispersion relation but over-massive compared to the BH mass-bulge luminosity relation by a factor of 3.7. PGC 11179 is part of a sample of local compact early-type galaxies that are plausible relics of z ∼ 2 red nuggets, and its behavior relative to the scaling relations echoes that of three relic galaxy BHs previously measured with stellar dynamics. These over-massive BHs could suggest that BHs gain most of their mass before their host galaxies do. However, our results could also be explained by greater intrinsic scatter at the high-mass end of the scaling relations, or by systematic differences in gas- and stellar-dynamical methods. Additional M_BH measurements in the sample, including independent cross-checks between molecular gas- and stellar-dynamical methods, will advance our understanding of the co-evolution of BHs and their host galaxies.