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

TL;DR

This paper predicts free-free absorption and emission properties in AGN gas influenced by radiation pressure, linking these to distance from the nucleus and comparing with observations, especially in NGC 1068.

Contribution

It provides new theoretical predictions for free-free absorption and emission in AGN, based on radiation pressure confinement models, and compares these with observational data.

Findings

Free-free absorption frequency increases from ~100 MHz to ~100 GHz from kpc to sub-pc scales.

Predicted radio loudness at 5 GHz is lower than typical in radio-quiet AGN.

Excess emission at >150 GHz in NGC 1068 aligns with free-free emission predictions.

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.