2022MNRAS.509.2920N

As part of the Measuring Black Holes in below Milky Way-mass (M^*) galaxies (MBHBM^*) Project, we present a dynamical measurement of the supermassive black hole (SMBH) mass in the nearby lenticular galaxy NGC 3593, using cold molecular gas ¹²CO(2-1) emission observed at an angular resolution of ≃0."3 (≃10 pc) with the Atacama Large Millimeter/submillimeter Array (ALMA). Our ALMA observations reveal a circumnuclear molecular gas disc (CND) elongated along the galaxy major axis and rotating around the SMBH. This CND has a relatively low-velocity dispersion (<=10 km s^–1) and is morphologically complex, with clumps having higher integrated intensities and velocity dispersions (<=25 km s^–1). These clumps are distributed along the ridges of a two-arm/bi-symmetric spiral pattern surrounded by a larger ring-like structure (radius r ≃ 10 arcsec or ≃350 pc). This pattern likely plays an important role to bridge the molecular gas reservoirs in the CND and beyond (10 <= r <= 35 arcsec or 350 pc <= r <= 1.2 kpc). Using dynamical modelling, the molecular gas kinematics allow us to infer an SMBH mass M_BH=2.40_–1.05^+1.87×10⁶ M_☉ (only statistical uncertainties at the 3σ level). We also detect a massive core of cold molecular gas (CMC) of mass M_CMC = (5.4 ± 1.2) x 10⁶ M_☉ and effective (half-mass) radius r_CMC,e = 11.2 ± 2.8 pc, co-spatial with a nuclear star cluster (NSC) of mass M_NSC = (1.67 ± 0.48) x 10⁷ M_☉ and effective radius r_NSC,e = 5.0 ± 1.0 pc (or 0."15 ± 0."03). The mass profiles of the CMC and NSC are well described by Sersic functions with indices 1-1.4. Our M_BH and M_NSC estimates for NGC 3593 agree well with the recently compiled M_BH-M_NSC scaling relation. Although the M_NSC uncertainty is twice the inferred M_BH, the rapid central rise of the rotation velocities of the CND (as the radius decreases) clearly suggests an SMBH. Indeed, our dynamical models show that even if M_NSC is at the upper end of its allowed range, the evidence for a BH does not vanish, but remains with a lower limit of M_BH > 3 x 10⁵ M_☉.