Precision Black Hole Masses
The authors present high resolution (0.1 arcsec ~ 15pc) ALMA imaging of the CO 2-1 emission of the dusty molecular disk surrounding the supermassive black hole (SMBH) in the massive elliptical galaxy, NGC3258 (d ~ 31 Mpc), probing well within the gravitational sphere of influence of the SMBH. These data reveal a quasi-Keplerian central increase in projected rotation speed rising from 280 km/sec at the disk's outer edge to >400 km/sec near the disk center. The authors construct dynamical models for the rotating disk and fit beam-smeared model CO line profiles directly to the ALMA data cube. These models incorporate flat and tilted-ring disks that provide a better fit of the mildly warped structure in NGC 3258, demonstrating 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. The 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 MBH = 2.249 x 109 M⊙, 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 black holes masses in early-type galaxies containing circumnuclear gas disks.
Image: [Left] CO 2-1 moment maps. [Right] CO 2-1 spectrum and PV diagram for NGC3258. (Boizelle et al. 2019ApJ...881...10B)
Publication: Benjamin D. Boizelle (Texas A&M, University of California, Irvine), et al., A Precision Measurement of the Mass of the Black Hole in NGC 3258 from High-resolution ALMA Observations of Its Circumnuclear Disk, Astrophysical Journal, 881, 10 (13 August 2019).
NRAO Press Release: ALMA Dives into Black Hole's 'Sphere of Influence'
