2021MNRAS.508..680B

The effect of radiation pressure compression (RPC) on ionized gas in active galactic nuclei (AGNs) likely sets the photoionized gas density structure. The photoionized gas free-free absorption and emission are therefore uniquely set by the incident ionizing flux. We use the photoionization code CLOUDY RPC model results to derive the expected relations between the free-free emission and absorption properties and the distance from the AGN centre, for a given AGN luminosity. The free-free absorption frequency of RPC gas is predicted to increase from ∼100 MHz on the kpc scale to ∼100 GHz on the sub-pc scale, consistent with observations of spatially resolved free-free absorption. The free-free emission at 5 GHz is predicted to yield a radio loudness (R) of ∼0.03, below the typical observed values of R ∼ 0.1-1 in radio-quiet AGNs. However, the flat free-free radio continuum may become dominant above 100 GHz. The suggested detection of optically thin free-free emission in NGC 1068, on the sub-pc torus scale, is excluded as the brightness temperature is too high for optically thin free-free emission. However, excess emission observed with Atacama Large Millimeter/submillimeter Array (ALMA) above 150 GHz in NGC 1068 is consistent with the predicted free-free emission from gas just outside the broad-line region, a region that overlaps the hot dust disc resolved with GRAVITY. Extended ∼100 pc-scale free-free emission is also likely present in NGC 1068. Future sub-mm observation of radio-quiet AGNs with ALMA may allow to image the free-free emission of warm photoionized gas in AGNs down to the 30 mas scale, including highly absorbed AGNs.