Radiation pressure confinement – V. The predicted free–free absorption and emission in active galactic nuclei

Abstract

ABSTRACT 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.