  The mechanism of radio emission in radio-quiet (RQ) active galactic nuclei
(AGN) is still debated and might arise from the central AGN, from star
formation activity in the host, or from either of these sources. A direct
detection of compact and bright radio cores embedded in sources that are
classified as RQ can unambiguously determine whether a central AGN
significantly contributes to the radio emission. We search for compact,
high-surface-brightness radio cores in RQ AGNs that are caused unambiguously by
AGN activity. We used the Australian Long Baseline Array to search for compact
radio cores in four RQ AGNs located in the Extended Chandra Deep Field South
(ECDFS). We also targeted four radio-loud (RL) AGNs as a control sample. We
detected compact and bright radio cores in two AGNs that are classified as RQ
and in one that is classified as RL. Two RL AGNs were not imaged because the
quality of the observations was too poor. We report on a first direct evidence
of radio cores in RQ AGNs at cosmological redshifts. Our detections show that
some of the sources that are classified as RQ contain an active AGN that can
contribute significantly (about 50% or more) to the total radio emission.
