2019ApJ...875...82H

We have examined a sample of 13 sub-Eddington supermassive black holes hosted by galaxies spanning a variety of morphological classifications to further understand the empirical fundamental plane of black hole activity. This plane describes black holes from stellar-mass to supermassive and relates the mass of an accreting black hole and its radio and X-ray luminosities. A key factor in studying the fundamental plane is the turnover frequency, i.e., the frequency at which the radio continuum emission becomes optically thin. We measured this turnover frequency using new Very Large Array observations combined, when necessary, with archival Chandra observations. Radio observations are in the range of 5-40 GHz across four frequency bands in B configuration, giving high spatial resolution to focus on the core emission. We use Markov chain Monte Carlo methods to fit the continuum emission in order to find the turnover frequency. After testing for correlations, the turnover frequency does not display a significant dependence on either the mass or mass accretion rate, indicating that more complicated physics than simple scaling and optical depth effects are at play, as has been suggested by recent theoretical work.