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

Abstract: The effect of radiation pressure compression (RPC) on ionized gas in Active Galactic Nuclei (AGN) 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 increases from $\sim$100 MHz on the kpc scale, to $\sim$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 of $R\sim 0.03$, below the typical observed values of $R\sim 0.1-1$ in radio-quiet AGN. 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 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 which overlaps the hot dust disc resolved with GRAVITY. Extended $\sim$100 pc scale free-free emission is also likely present in NGC 1068. Future sub mm observation of radio quiet AGN with ALMA may allow to image the free-free emission of warm photoionized gas in AGN down to the 30 mas scale, including highly absorbed AGN.