Gas-dynamical mass measurement of the supermassive black hole in the local compact galaxy UGC 2698

Supermassive black holes (BHs) play a key role in galaxy evolution. One fundamental open question is whether BHs and galaxies grow in lockstep, or if the growth of one precedes that of the other. We have uncovered a sample of compact galaxies that are thought to be local analogs of z~2 quiescent galaxies. These local (z ≤ 0.02) systems have uniformly old (≥10 Gyr) stellar populations and small sizes (effective radii of 0.7-3.1 kpc) for their stellar masses [(0.5-3.8)×10¹¹ solar masses]. Previous stellar-dynamical modeling of several of these galaxies found over-massive BHs relative to the BH mass - bulge luminosity relation, suggesting that perhaps BHs build the majority of their mass before their host galaxies. Here, we present 0.20"-resolution Atacama Large Millimeter/submillimeter Array (ALMA) CO(2-1) observations of the rotating circumnuclear disk in the local compact galaxy UGC 2698. We fit gas-dynamical models directly to the ALMA data cube, assuming the CO emission originates from a dynamically cold, thin disk, and we use the dynamic nested sampling code dynesty to determine Bayesian posteriors. We find a black hole mass in UGC 2698 of 2.46×10⁹ solar masses, with an error budget dominated by an estimated systematic uncertainty of ~30%. We discuss our results and locate UGC 2698 relative to the BH - host galaxy relationships, finding it is consistent with the local scaling relations. Expanding the sample of local compact galaxies with dynamical BH mass measurements will pave the way for a deeper understanding of BH - galaxy co-evolution.