VLBI Observations of a sample of Palomar-Green quasars II: characterising the parsec-scale radio emission

TL;DR

This study employs multi-frequency VLBI to analyze the parsec-scale radio emission in a sample of radio-quiet and radio-loud quasars, revealing the origins and characteristics of their nuclear radio activity.

Contribution

It provides new insights into the nature of radio emission in RQQs, highlighting the roles of compact jets, corona, and star formation, and compares these with RLQs using high-resolution VLBI data.

Findings

RQQs' radio emission mainly from compact jets or corona

VLBI cores in RQQs are less dominant than in RLQs

Variability and spectral properties distinguish emission types

Abstract

This study uses multi-frequency Very Long Baseline Interferometry (VLBI) to study the radio emission from 10 radio-quiet quasars (RQQs) and four radio-loud quasars (RLQs). The diverse morphologies, radio spectra, and brightness temperatures observed in the VLBI images of these RQQs, together with the variability in their GHz spectra and VLBI flux densities, shed light on the origins of their nuclear radio emission. The total radio emission of RQQs appears to originate from non-thermal synchrotron radiation due to a combination of active galactic nuclei and star formation activities. However, our data suggest that the VLBI-detected radio emission from these RQQs is primarily associated with compact jets or corona, with extended emissions such as star formation and large-scale jets being resolved by the high resolution of the VLBI images. Wind emission models are not in complete agreement the VLBI observations. Unlike RLQs, where the parsec-scale radio emission is dominated by a relativistically boosted core, the radio cores of RQQs are either not dominant or are mixed with significant jet emission. RQQs with compact cores or core-jet structures typically have more pronounced variability, with flat or inverted spectra, whereas jet-dominated RQQs have steep spectra and unremarkable variability. Future high-resolution observations of more RQQs could help to determine the fraction of different emission sources and their associated physical mechanisms.