My research focuses on precision measurement of supermassive black hole masses.
Supermassive Black Holes in Nearby Galaxies
A supermassive black hole (BH), with a mass between about a million to several billion times the mass of the Sun, is found at the center of nearly every large galaxy. While amounting to only a small fraction of their host galaxy's total mass, these BHs play an oversized role in galaxy formation and evolution. Within the so-called "sphere of influence" that can be hundreds of light-years across, the BH dominates over the enclosed stellar, gaseous, and dark matter mass. Astronomers constrain a BH's mass by measuring and modeling the motion of stars or gas clouds that probe this region's gravitational potential. For even the nearest galaxies, however, this region is typically much less than an arcsecond across. Modern telescopes like the Hubble Space Telescope (HST) and the Atacama Large Millimeter/submillimeter Array (ALMA) can resolve kinematic tracers within the sphere of influence of many nearby galaxies and have enabled about 100 BH mass measurements over the past 30 years.
The BH census remains incomplete for the most massive galaxies, with the handful of BH mass measurements in luminous early-type galaxies (ETGs; comprising elliptical and lenticular types) being accompanied by large uncertainties. As an early career astronomer, I have focused on obtaining precision BH mass measurements in very massive galaxies to better understand the growth of these compact objects in the most extreme galaxy merger environments.
Cold Molecular Disks with ALMA
The high sensitivity and angular resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) opens a new and exciting avenue to measure BH masses. Gaseous disks are found at the centers of many galaxies, with the accompanying dust obscuring background visible stellar light while the CO molecule emits light at specific mm/sub-mm wavelengths covered by ALMA. Past surveys have found these round, circumnuclear gas disks to be in regular rotation down to galaxy nuclei. In about 10% of all nearby ETGs, HST images reveal circumnuclear disks that make these galaxies prime candidates for BH mass measurements using ALMA.
In ALMA Cycle 2, we began a program to measure molecular gas kinematics in and around the sphere of influence in ~25 ETGs. The molecular gas (traced by CO emission) corresponds well with the dust morphology and is in dynamically cold rotation about their galaxy centers (Boizelle et al. 2017). Adding in ALMA observations taken by other teams, circumnuclear disks have now been mapped in roughly 50 ETGs.
Image (above): Maps of integrated line flux (left) and line-of-sight velocity (right) from CO imaging of dust-disk ETGs obtained from the ALMA archive. This sample reveals a wide range of gas morphologies, including rings and central holes in CO emission, with some small level of kinematic twists in most cases. Synthesized beam shapes are shown by the gray ellipses next to the flux maps, and the range of Doppler red and blue-shifted velocities in km/s are indicated in each velocity map. (For more information on individual data sets, see: Boizelle et al. 2017; Davis et al. 2017; Davis et al. 2018; Smith et al. 2019; North et al. 2019; Ruffa et al. 2019; Rose et al. 2019; Boizelle et al. 2019; Davis et al. 2020)
Our initial data were obtained with an angular resolution to allow the identification of any rapid gas rotation very near the galaxy center. When this "smoking gun" high-velocity gas is observed, we obtain follow-up high-resolution ALMA observations to enable black hole mass measurements with exquisite precision.
Image (right): Integrated line flux and line-of-sight velocity maps from ALMA CO(2-1) imaging of NGC 3258 alongside those measured from the best-fitting model cube. This detailed modeling reaches unprecedented modeling precision (less than 1%) since these observations highly resolve the BH's sphere of influence. Adapted from Boizelle et al. 2019.