Modeling ALMA Observations of the Warped Molecular Gas Disk in the Red Nugget Relic Galaxy NGC 384

Abstract

We present 0.″22 resolution CO(2–1) observations of the circumnuclear gas disk in the local compact galaxy NGC 384 with the Atacama Large Millimeter/submillimeter Array (ALMA). While the majority of the disk displays regular rotation with projected velocities rising to 370 km s^‑1, the inner ∼0.″5 exhibits a kinematic twist. We develop warped disk gas-dynamical models to account for this twist, fit those models to the ALMA data cube, and find a stellar mass-to-light ratio in the H band of M/L _H = 1.34 ± 0.01 [1σ statistical] ±0.02 [systematic] M _⊙/L _⊙ and a supermassive black hole (BH) mass (M _BH) of M _BH =(7.26‑0.48+0.43[1σstatistical]‑1.00+0.55[systematic])×108M⊙ . In contrast to most previous dynamical M _BH measurements in local compact galaxies, which typically found over-massive BHs compared to the local BH mass‑bulge luminosity and BH mass‑bulge mass relations, NGC 384 lies within the scatter of those scaling relations. NGC 384 and other local compact galaxies are likely relics of z ∼ 2 red nuggets, and over-massive BHs in these relics indicate BH growth may conclude before the host galaxy stars have finished assembly. Our NGC 384 results may challenge this evolutionary picture, suggesting there may be increased scatter in the scaling relations than previously thought. However, this scatter could be inflated by systematic differences between stellar- and gas-dynamical measurement methods, motivating direct comparisons between the methods for NGC 384 and the other compact galaxies in the sample.