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

Unveiling the origin of the radio emission in radio-quiet quasars