VLBI Observations of a sample of Palomar-Green quasars I: parsec-scale morphology

TL;DR

This study uses VLBA observations at 5 GHz to investigate the parsec-scale radio morphology of a sample of Palomar-Green quasars, revealing active galactic nucleus-related radio emission in many radio-quiet quasars and differences between radio-quiet and radio-loud quasars.

Contribution

First high-resolution VLBA imaging of a flux-limited PG quasar sample, highlighting the prevalence of AGN-related radio emission in RQQs and differences in morphology compared to RLQs.

Findings

Active galactic nucleus-related radio emission detected in most RQQs.

RQQs show more extended jets or relic jets than RLQs.

Significant morphological and flux density differences between RQQs and RLQs.

Abstract

We observed 20 Palomar-Green (PG) quasars at low redshift ($z<0.5$) with total flux density > 1 mJy, including 4 radio-loud quasars (RLQs) and 16 radio-quiet quasars (RQQs), using the Very Long Baseline Array (VLBA) at 5 GHz. Ten RQQs are clearly detected in the VLBA images, and a compact radio core is identified in eight of them, indicating the prevalence of active galactic nucleus (AGN)-related radio emission in this flux-density-limited RQQ sample. The RQQs and RLQs in our sample have a division at $\sim$30~mJy. The radio emission from RQQs appears to be the result of a combination of star formation and AGN-associated activities. All RQQs in our sample have a 5 GHz flux density ratio of Very Large Array (VLA) A-array to D-array $f_{\rm c} = S_{\rm A}^{\rm VLA}/S_{\rm D}^{\rm VLA}$ above 0.2. The RQQs with $f_a$ (VLBA and VLA flux density ratio $S^{\rm VLBA}/S_{\rm A}^{\rm VLA}) > 0.2$ versus $f_a < 0.2$ show significant differences in morphology, compactness and total flux density. $f_{\rm a}$ of RQQs is systematically lower than that of RLQs, probably due to the extended jets or relic jets of RQQs on 10s to 100s parsecs which are resolved out in VLBA images. Future larger samples, especially with the addition of milli-arcsec resolution radio images of RQQs with total flux densities below 1 mJy, can test the conclusions of this paper and contributes to the understanding of the radio emission mechanism of RQQs, and the dichotomy and physical connection between RQQs and RLQs.