Constraining the Mass of M95's Supermassive Black Hole with ALMA

Abstract

We attempt to constrain the mass of the supermassive black hole (SMBH) in the center of the nearby late-type, quiescent, cold gas galaxy M95 (NGC 3351) using sub-millimeter CO emission line observations from the Atacama Large Millimeter Array (ALMA). The formation and development of galaxies and the mass and growth of SMBHs are closely tied, and the effectiveness of using SMBH masses to model galaxy evolution is highly dependent on how accurate the measurements are. This makes high-resolution sub-millimeter CO observations incredibly useful to accurately estimate the SMBH mass of late-type, cold gas-rich galaxies with a quiescent nucleus (where alternative methods with tracers other than CO cannot do so). Our dataset has a beam size of 0.1'' resolution with a 2.5 km/s spectral channel width, with a low noise floor of 1.5 K. Our CO(3-2) line detections near the black hole have integrated intensities near 2,000 K km/s, making our S/N ratio in the this region very high for modelling it against background. We used KinMS (KINematic Molecular Simulation) to model the central molecular torus surrounding M95's central SMBH, and after creating two models (one with a Keplerian velocity profile component and one without) and choose the results of the model that we are most confident matches the data. From this, we determined a hard upper limit that the mass of M95's central supermassive black hole is no greater than 6.543 x 10^5 solar masses given that we estimate that there is no more than that amount of dynamical mass within 0.1'' (or about ~5 pc) of the center of M95. We also determined a (less strict) upper limit that all central Keplerian mass in the center of M95 is at most ~3 x 10^5 solar masses (which could contain objects and structures other than the SMBH). Both upper limits are smaller than we would expect given the confirmed central SMBH masses of other galaxies, which gives us an interesting result that can be used to study how M95 might evolve differently from similar galaxies with classical bulges (which M95 does not have).