Unveiling the origin of the radio emission in radio-quiet quasars
  The origin of the radio emission in radio-quiet quasars (RQQs) has been a
matter of debate for a long time. It is not well understood whether the
emission is caused by star formation in the host galaxy or by black hole
activity of the active galactic nuclei (AGN). We shed some light on these
questions using the Very Long Baseline Interferometry (VLBI) technique to
search for RQQs in the field of the Cosmological Evolution Survey (COSMOS). The
extensive multi-wavelength coverage of the field (from radio to X-rays) was
used to classify RQQs, and the milli-arcsecond resolution of VLBI provides a
direct way to identify AGNs. In a sample of 18 RQQs we detected 3 using the
Very Long Baseline Array (VLBA) at 1.4 GHz. In this letter we report for the
first time on a sample of RQQs with a measured lower limit on the fraction of
radio emission coming from the AGN, thus demonstrating that the radio emission
of at least some RQQs is dominated by an AGN.
