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

Abstract

We conduct kinematic analysis of the SDSS spectra of 568 obscured luminous quasars, with the emphasis on the kinematic structure of the [OIII]5007 emission line. [OIII] emission tends to show blueshifts and blue excess, which indicates that at least part of the narrow-line gas is undergoing an organized outflow. The velocity width containing 90% of line power ranges from 370 to 4780 km/sec, suggesting outflow velocities up to 2000 km/sec. The velocity width of the [OIII] emission is positively 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[1.4GHz] = 10^40 erg/sec suggests a median kinetic luminosity of the quasar-driven wind of L_wind=3x10^44 erg/sec, or about 4% of the estimated median bolometric luminosity L_bol=8x10^45 erg/sec. 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. As the outflow velocity increases, some emission lines characteristic of shocks in quasi-neutral medium increase as well, which we take as further evidence of quasar-driven winds propagating into the interstellar medium of the host galaxy. None of the kinematic components show correlations with the stellar velocity dispersions of the host galaxies, so there is no evidence that any of the gas in the narrow-line region of quasars is in dynamical equilibrium with the host galaxy. Quasar feedback appears to operate above the threshold luminosity of L_bol=3x10^45 erg/sec.