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

¹ Astronomisches Institut, Ruhr-Universität Bochum, Universitätstrasse 150, 44801 Bochum, Germany
e-mail: herrera@astro.rub.de
² CSIRO Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia
³ Western Sydney University, Locked Bag 1797, Penrith South, NSW 1797, Australia
⁴ Dipartimento di Fisica e Astronomia, Università di Bologna, viale B. Pichat 6/2, 40127 Bologna, Italy
⁵ Department of Physics and Astronomy, Macquarie University, Balaclava Road, North Ryde, NSW 2109, Australia
⁶ INAF–IRA, via P. Gobetti 101, 40129 Bologna, Italy

Received: 12 February 2016
Accepted: 12 March 2016

Abstract

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.