A consistent radio to sub-mm pc-scale study of the nucleus of NGC 1068

The origin of radio emission in radio-quiet (RQ) active galactic nuclei (AGN) remains a long-standing mystery. We present a detailed study of the cm to sub-mm emission from the nucleus of the nearby prototypical RQ Seyfert 2 galaxy, NGC 1068. We analyse observations between 4.5 and 706 GHz using e-MERLIN, VLA, and ALMA. We restricted all data used for imaging to a matching uv-range of 15–3300 kλ, to ensure that all data sampled the same spatial scales. All images were restored with a ∼0.06 arcsec beam. To derive the spectral energy distribution (SED), we fit synchrotron, free–free, dust, and coronal component models to these data. We report that the sub-mm excess between ∼200 and 700 GHz is consistent with synchrotron emission from a compact and optically thick corona with a radius Rc ≈ 70 ± 5 Rg, a fraction of ∼10 ± 2 % of the energy density in the form of non-thermal electrons, and a magnetic field strength B ≈ 148 G. The luminosity of the corona is roughly consistent with—though higher than—the expected value from mm–X-ray correlations derived in recent studies of RQ AGNs. This difference is likely due to the corona SED peaking at ≈ 550 GHz. Between 10 and ∼200 GHz, the SED is dominated by free–free emission. High-angular-resolution observations at frequencies below 5 GHz are needed to constrain a putative optically thin synchrotron component and the amount of free–free absorption.