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The origin of the radio emission in radio-quiet quasars (RQQs) remainsunclear. Radio emission may be produced by a scaled-down version of therelativistic jets observed in radio-loud (RL) AGN, an AGN-driven wind, theaccretion disc corona, AGN photon-ionisation of ambient gas (free-freeemission), or star formation (SF). Here, we report a pilot study, part of aradio survey (`PG-RQS') aiming at exploring the spectral distributions of the71 Palomar-Green (PG) RQQs: high angular resolution observations ($\sim$50 mas)at 45~GHz (7 mm) with the Karl G. Jansky Very Large Array of 15 sources.Sub-mJy radio cores are detected in 13 sources on a typical scale of $\sim$100pc, which excludes significant contribution from galaxy-scale SF. For 9 sourcesthe 45-GHz luminosity is above the lower frequency ($\sim$1-10 GHz) spectralextrapolation, indicating the emergence of an additional flatter-spectrumcompact component at high frequencies. The X-ray luminosity and black hole (BH)mass, correlate more tightly with the 45-GHz luminosity than the 5-GHz. The 45GHz-based radio-loudness increases with decreasing Eddington ratio andincreasing BH mass M$_{\rm BH}$. These results suggest that the 45-GHz emissionfrom PG RQQs nuclei originates from the innermost region of the core, probablyfrom the accretion disc corona. Increasing contributions to 45-GHz emissionfrom a jet at higher M$_{\rm BH}$ and lower Eddington ratios and from a discwind at large Eddington ratios are still consistent with our results. Futurefull radio spectral coverage of the sample will help us investigating thedifferent physical mechanisms in place in RQQ cores.

The PG-RQS survey. Building the radio spectral distribution of radio-quiet quasars. I. The 45-GHz data