Using strong lensing to understand the microJy radio emission in two radio quiet quasars at redshift 1.7

TL;DR

This study uses strong gravitational lensing and VLA radio observations to investigate the origin of radio emission in two radio-quiet quasars at redshift 1.7, providing evidence for AGN-related activity in faint sources.

Contribution

It presents the first detailed radio analysis of strongly lensed RQQs at high redshift, revealing AGN-related radio emission in sources previously thought to be star-formation dominated.

Findings

Evidence of AGN-related radio variability in SDSS J1004+4112

Tentative AGN emission indication in PG 1115+080

Faintest radio sources imaged showing AGN-dominated emission

Abstract

The radio quasar luminosity function exhibits an upturn around $L_{6\rm\:GHz}=10^{23}$ W Hz$^{-1}$ that is well-modelled by a star-forming host galaxy population. This distribution leads some authors to cite star formation as the main radio emission mechanism in so-called radio-quiet quasars (RQQs). Understanding the origin of RQQ radio emission is crucial for our understanding of quasar feedback mechanisms -- responsible for the regulation of star-formation in the host galaxy -- and for understanding galaxy evolution as a whole. By observing RQQs that have been magnified by strong gravitational lensing, we have direct access to the RQQ population out to cosmic noon, where evidence for twin mini-jets has recently been found in a sub-\textmu Jy RQQ. Here we present radio observations of two lensed RQQs using the VLA at 5~GHz, the latest objects to be observed in a sample of quadruply-imaged RQQs above -30$^{\circ}$. In SDSS~J1004+4112 we find strong evidence for AGN-related radio emission in the variability of the source. In PG~1115+080 we find tentative evidence for AGN-related emission, determined by comparing the radio luminosity with modelled dust components. If confirmed in the case of PG~1115+080, which lies on the radio--FIR correlation, the result would reinforce the need for caution when applying the correlation to rule out jet activity and when assuming no AGN heating of FIR-emitting dust when calculating star formation rates. Our programme so far has shown that two of the faintest radio sources ever imaged show strong evidence for AGN-dominated radio emission.