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

Abstract

We analyse Sloan Digital Sky Survey spectra of 568 obscured luminous quasars. The [O III] lambda 5007 angstrom emission line shows blueshifts and blue excess, indicating that some of the narrow-line 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 similar 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 nu L-nu [1.4 GHz] = 10(40) erg s(-1) suggests a median kinetic luminosity of the quasar-driven wind of L-wind = 3 x 10(44) erg s(-1), or about 4 per cent of the estimated median bolometric luminosity L-bol = 8 x 10(45) erg s(-1). Furthermore, the velocity width of [O III] 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 L-bol similar to 3 x 10(45) erg s(-1).