Kpc-scale Radio Structure in $z\sim0.25$ Radio-Quiet QSOs

Abstract

We present analysis of a homogeneous, optically selected, volume-limited ($0.2<z<0.3$) sample of 128 radio-quiet quasi-stellar objects (QSOs) recently observed at 6 GHz with the Very Large Array (VLA) in A-configuration ($\sim0.33''$ resolution). We compare these new results to earlier (2010--2011) 6-GHz observations with the VLA in C-configuration ($\sim3.5''$). While all of these radio-quiet QSOs (RQQs) were unresolved on a $3.5''$ scale ($\sim$14 kpc at $z=0.25$), we resolve notable complex sub-galactic structures in about half of the RQQs at $0.33''$ resolution ($\sim$1.3 kpc at $z=0.25$). By comparison of flux density measurements between the two sets of observations, we demonstrate that significant sub-galactic-scale radio structure is present in at least 70% of the RQQ population, and that the central component accounts for an average of $\approx$65% of the total detected radio power. One RQQ, J0935+4819, shows striking symmetric, double-lobed morphology, and appears to be the first identified example of a radio-$\mathrm{\textit{quiet}}$ QSO with FR II type morphology on $\sim$arcsec scale (projected size of $\gtrsim6$ kpc). In addition to revealing RQQ sub-galactic morphology, we employ counterparts from legacy (FIRST at 1.4 GHz) and recent (VLA Sky Survey at 3 GHz) VLA surveys to investigate radio spectral indices and potential variability over decades-long timescales for a subset of the RQQs, and for the cores of radio-intermediate and -loud sources in the parent sample of 178 QSOs. These results support the growing notion that the RQQ population is not a monolithic phenomenon, but instead consists of a mixture of mainly starburst-powered and jet-powered galaxies.