2019MNRAS.485.3009H

We present e-MERLIN and European VLBI Network observations which reveal unambiguous jet activity within radio-quiet quasar HS 0810+2554. With an intrinsic flux density of 880 nJy, this is the faintest radio source ever imaged. The findings present new evidence against the idea that radio-loud and radio-quiet quasars are powered by different underlying radio emission mechanisms, showing instead that the same active galactic nucleus (AGN) mechanism can operate as the dominant source of radio emission even in the very lowest radio luminosity quasars. Due to strong gravitational lensing, our source is not only visible, but with VLBI is imaged to a scale of just 0.27 pc: the highest ever resolution image of a radio-quiet quasar. Brightness temperatures of at least 8.4 x 10⁶ K are associated with two highly compact components. Subsequent modelling of the lensed system has revealed that the components are linearly aligned on opposing sides of the optical quasar core, with the typical morphology of a compact symmetric object (CSO). Given that this source has been found to fall on the radio-FIR correlation, we suggest that the radio-FIR correlation cannot always be used to rule out AGN activity in favour of star formation activity. The correlation - or at least its scatter - may conceal the coexistence of kinetic and radiative feedback modes in AGN. Modelling of the lensing mass itself points to a non-smooth mass distribution, hinting at the presence of dark matter substructure which has manifested as astrometric perturbations of the VLBI lensed images.