We present ~0farcs10 resolution Atacama Large Millimeter/submillimeter Array (ALMA) CO(2−1) imaging of the arcsecond-scale (r ≈ 150 pc) dusty molecular disk in the giant elliptical galaxy NGC 3258. The data provide unprecedented resolution of the cold gas disk kinematics within the dynamical sphere of influence of a supermassive black hole (BH), revealing a quasi-Keplerian central increase in projected rotation speed rising from 280 km s−1 at the disk's outer edge to >400 km s−1 near the disk center. We construct dynamical models for the rotating disk and fit beam-smeared model CO line profiles directly to the ALMA data cube. Our models incorporate both flat and tilted-ring disks that provide a better fit of the mildly warped structure in NGC 3258. We show that the exceptional angular resolution of the ALMA data makes it possible to infer the host galaxy's mass profile within r = 150 pc solely from the ALMA CO kinematics, without relying on optical or near-infrared imaging data to determine the stellar mass profile. Our model therefore circumvents any uncertainty in the BH mass that would result from the substantial dust extinction in the galaxy's central region. The best model fit yields ${M}_{\mathrm{BH}}=2.249\times {10}^{9}$ ${M}_{\odot }$, with a statistical model-fitting uncertainty of just 0.18% and systematic uncertainties of 0.62% from various aspects of the model construction and 12% from uncertainty in the distance to NGC 3258. This observation demonstrates the full potential of ALMA for carrying out highly precise measurements of ${M}_{\mathrm{BH}}$ in early-type galaxies containing circumnuclear gas disks.

Emission line observations of circumnuclear gas disks in the ALMA era have begun to resolve molecular gas tracer kinematics near supermassive black holes (BHs), enabling highly precise mass determination in the best cases. The ngVLA is capable of extremely high spatial resolution imaging of the CO(1–0) transition at 115 GHz for nearby galaxies. Furthermore, its high (anticipated) emission line sensitivity suggests this array can produce benchmark BH mass measurements. We discuss lessons learned from gas-dynamical modeling of recent ALMA data sets and also compare ALMA and ngVLA CO simulations of a dynamically cold disk. While only a fraction of all local galaxies likely possess sufficiently bright, regularly-rotating nuclear molecular gas, in such cases the ngVLA is expected to more efficiently resolve such emission arising at a projected 50–100 mas from the central BH.

We present results from an Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 2 program to map CO(2−1) emission in nearby early-type galaxies (ETGs) that host circumnuclear gas disks. We obtained ~0farcs3 resolution Band 6 observations of seven ETGs selected on the basis of dust disks in Hubble Space Telescope images. We detect CO emission in five at high signal-to-noise ratio with the remaining two only faintly detected. All CO emission is coincident with the dust and is in dynamically cold rotation. Four ETGs show evidence of rapid central rotation; these are prime candidates for higher-resolution ALMA observations to measure the black hole masses. In this paper, we focus on the molecular gas and continuum properties. Total gas masses and H2 column densities for our five CO-bright galaxies are on average ~108 M ☉ and $\sim {10}^{22.5}$ cm−2 over the ~kpc-scale disks, and analysis suggests that these disks are stabilized against gravitational fragmentation. The continuum emission of all seven galaxies is dominated by a central unresolved source, and in five we also detect a spatially extended component. The ~230 GHz nuclear continua are modeled as power laws ranging from ${S}_{\nu }\sim {\nu }^{-0.4}$ to ${\nu }^{1.6}$ within the observed frequency band. The extended continuum profiles of the two radio-bright (and CO-faint) galaxies are roughly aligned with their radio jet and suggest resolved synchrotron jets. The extended continua of the CO-bright disks are coincident with optically thick dust absorption and have spectral slopes that are consistent with thermal dust emission.

We present first results from a program of Atacama Large Millimeter/submillimeter Array (ALMA) CO(2–1) observations of circumnuclear gas disks in early-type galaxies. The program was designed with the goal of detecting gas within the gravitational sphere of influence of the central black holes (BHs). In NGC 1332, the 0farcs3-resolution ALMA data reveal CO emission from the highly inclined ($i\approx 83^\circ $) circumnuclear disk, spatially coincident with the dust disk seen in Hubble Space Telescope images. The disk exhibits a central upturn in maximum line-of-sight velocity, reaching ±500 km s−1 relative to the systemic velocity, consistent with the expected signature of rapid rotation around a supermassive BH. Rotational broadening and beam smearing produce complex and asymmetric line profiles near the disk center. We constructed dynamical models for the rotating disk and fitted the modeled CO line profiles directly to the ALMA data cube. Degeneracy between rotation and turbulent velocity dispersion in the inner disk precludes the derivation of strong constraints on the BH mass, but model fits allowing for a plausible range in the magnitude of the turbulent dispersion imply a central mass in the range of ~(4–8) × 108 ${M}_{\odot }$. We argue that gas-kinematic observations resolving the BH's projected radius of influence along the disk's minor axis will have the capability to yield BH mass measurements that are largely insensitive to systematic uncertainties in turbulence or in the stellar mass profile. For highly inclined disks, this is a much more stringent requirement than the usual sphere-of-influence criterion.

We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 3 observations of CO(2–1) emission from the circumnuclear disk in the E/S0 galaxy NGC 1332 at 0farcs044 resolution. The disk exhibits regular rotational kinematics and central high-velocity emission (±500 km s−1) consistent with the presence of a compact central mass. We construct models for a thin, dynamically cold disk in the gravitational potential of the host galaxy and black hole and fit the beam-smeared model line profiles directly to the ALMA data cube. Model fits successfully reproduce the disk kinematics out to r = 200 pc. Fitting models just to spatial pixels within projected r = 50 pc of the nucleus (two times larger than the black hole's gravitational radius of influence), we find ${M}_{\mathrm{BH}}=({6.64}_{-0.63}^{+0.65})\times {10}^{8}$ ${M}_{\odot }$. This observation demonstrates ALMA's powerful capability to determine the masses of supermassive black holes by resolving gas kinematics on small angular scales in galaxy nuclei.