The PG-RQS survey. Building the radio spectral distribution of
  radio-quiet quasars. I. The 45-GHz data
  The origin of the radio emission in radio-quiet quasars (RQQs) remains
unclear. Radio emission may be produced by a scaled-down version of the
relativistic jets observed in radio-loud (RL) AGN, an AGN-driven wind, the
accretion disc corona, AGN photon-ionisation of ambient gas (free-free
emission), or star formation (SF). Here, we report a pilot study, part of a
radio survey (`PG-RQS') aiming at exploring the spectral distributions of the
71 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$100
pc, which excludes significant contribution from galaxy-scale SF. For 9 sources
the 45-GHz luminosity is above the lower frequency ($\sim$1-10 GHz) spectral
extrapolation, indicating the emergence of an additional flatter-spectrum
compact component at high frequencies. The X-ray luminosity and black hole (BH)
mass, correlate more tightly with the 45-GHz luminosity than the 5-GHz. The 45
GHz-based radio-loudness increases with decreasing Eddington ratio and
increasing BH mass M$_{\rm BH}$. These results suggest that the 45-GHz emission
from PG RQQs nuclei originates from the innermost region of the core, probably
from the accretion disc corona. Increasing contributions to 45-GHz emission
from a jet at higher M$_{\rm BH}$ and lower Eddington ratios and from a disc
wind at large Eddington ratios are still consistent with our results. Future
full radio spectral coverage of the sample will help us investigating the
different physical mechanisms in place in RQQ cores.
