2024MNRAS.529.1597D

Supermassive black hole (SMBH) masses can be measured by observing their dynamical effects on tracers, such as molecular gas. We present high angular resolution Atacama Large Millimeter/submillimeter Array observations of the ¹²CO(2-1) line emission of the early-type galaxies (ETGs) NGC 1684 and NGC 0997, obtained as part of the MASSIVE survey, a volume-limited integral-field spectroscopic study of the most massive local ETGs. NGC 1684 has a regularly rotating central molecular gas disc, with a spatial extent of ≈6 arcsec (≈1.8 kpc) in radius and a central hole slightly larger than the expected SMBH sphere of influence. We forward model the data cube in a Bayesian framework with the KINEMATIC MOLECULAR SIMULATION (KINMS) code and infer a SMBH mass of $1.40^{+0.44}_{-0.39}\times 10^9$ M_⊙ (3σ confidence interval) and an F110W-filter stellar mass-to-light ratio of (2.50 ± 0.05) M_⊙/L_⊙, F110W_. NGC 0997 has a regularly rotating central molecular gas disc, with a spatial extent of ≈5 arcsec (≈2.2 kpc) in radius and a partially filled central hole much larger than the expected SMBH sphere of influence, thus preventing a robust SMBH mass determination. With the same modelling method, we nevertheless constrain the SMBH mass to be in the range 4.0 × 10⁷-1.8 × 10⁹ M_⊙ and the F160W-filter stellar mass-to-light ratio to be (1.52 ± 0.11) M_⊙/L_⊙, F160W_. Both SMBH masses are consistent with the SMBH mass-stellar velocity dispersion (M_BH-σ_e) relation, suggesting that the overmassive SMBHs present in other very massive ETGs are fairly uncommon.