Quasar feedback and the origin of radio emission in radio-quiet quasars

We analyse Sloan Digital Sky Survey spectra of 568 obscured luminous quasars. The [OIII] λ5007 Å emission line shows blueshifts and blue excess, indicating that some of the narrowline gas is undergoing an organized outflow. The velocity width containing 90 per cent of line power ranges from 370 to 4780 km s^-1, suggesting outflow velocities up to ~2000 km s^-1, and is strongly correlated with the radio luminosity among the radio-quiet quasars. We propose that radio emission in radio-quiet quasars is due to relativistic particles accelerated in the shocks within the quasar-driven outflows; star formation in quasar hosts is insufficient to explain the observed radio emission. The median radio luminosity of the sample of vL_v[1.4 GHz] = 10⁴⁰ erg s^-1 suggests a median kinetic luminosity of the quasar-driven wind of L_wind = 3 × 10⁴⁴ erg s^-1, or about 4 per cent of the estimated median bolometric luminosity L_bol = 8 × 10⁴⁵ erg s^-1. Furthermore, the velocity width of [OIII] is positively correlated with mid-infrared luminosity, which suggests that outflows are ultimately driven by the radiative output of the quasar. Emission lines characteristic of shocks in quasi-neutral medium increase with the velocity of the outflow, which we take as evidence of quasar-driven winds propagating into the interstellar medium of the host galaxy. Quasar feedback appears to operate above the threshold luminosity of Lbol ~ 3 × 10⁴⁵ erg s^-1.