Investigating Circumnuclear Radio Emission Mechanisms in Hard X-ray Selected Active Galactic Nuclei
Open Access DepositedRecent observations by the Fundamental Reference Active Galactic Nucleus (AGN)Monitoring Experiment (FRAMEx) explored radio quiet AGNs with the Very Long Baseline Array (VLBA) to understand the source of radio emission for a volume- complete (< 40 Mpc) sample of 25 hard X-ray selected AGNs. The sample has previously been detected in archival radio data taken with the Very Large Array (VLA) but only 12 out of 25 sources were detected at subparsec spatial scales when probed by the VLBA observations, despite expectations. In this dissertation, I ana- lyze the∼100 pc spatial scales of these AGNs at the resolution limits of the recently upgraded VLA to explore potential radiation mechanisms that may lead to the de- tection discrepancy. I used a newly obtained uniform set of 4−12 GHz observations for all 25 FRAMEx targets with the VLA in its full-Stokes polarization mode and I describe the calibration, analysis, and imaging of these broadband observations. I generated detailed imaging of the AGNs and their radio environments and I uniquely measured 4−12 GHz spectral energy distributions for the nuclei of all targets in the sample. I find that the AGNs without VLBA detections have spectral indices indicative of optically thin synchrotron emission while the VLBA-detected objects have the brightest radio emission at VLA resolution and show signatures of spectral curvature. I consider these observations in the context of whether the radio emission could be dominated by coronal emission, jet activity, AGN winds, or star forming processes. I find that radio emission origins remain ambiguous for a majority of the targets if it were entirely due to coronal or jet activity. I demonstrate that the radio emission is likely too luminous for star formation alone and that AGN winds are a viable mechanism of radio emission produced in situ from an energetics perspective.
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