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

Philippa Hartley, N. Jackson+4 more|arXiv (Cornell University)|Sep 22, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy169 references16 citations

TL;DR

This study uses strong gravitational lensing to probe the origin of microjansky radio emission in two radio-quiet quasars at z ≈ 1.7. By observing with the VLA at 5 GHz, it finds strong evidence for AGN-dominated radio emission in SDSS J1004+4112 via variability and tentative evidence in PG 1115+080 via radio–FIR correlation analysis, challenging assumptions that such faint radio emission is primarily star-formation-driven.

ABSTRACT

The radio quasar luminosity function exhibits an upturn around $L_{6 m\:GHz}=10^{23}$ 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- extmu 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.

Motivation & Objective

• To determine the origin of faint microjansky-level radio emission in radio-quiet quasars (RQQs), which remains debated between AGN jets and star formation.
• To test whether the radio–far-infrared (radio–FIR) correlation can reliably distinguish between AGN and star formation as the dominant radio emission mechanism in RQQs.
• To use strong gravitational lensing to access high-resolution, high-sensitivity radio observations of intrinsically faint RQQs, enabling direct study of their radio properties at cosmic noon.
• To investigate the role of host galaxy morphology and AGN feedback mechanisms in shaping radio emission in RQQs.
• To constrain dark matter substructure in lensing systems using flux ratio anomalies in lensed quasar images.

Proposed method

• Utilized the Very Large Array (VLA) in A configuration to observe two strongly lensed RQQs—SDSS J1004+4112 and PG 1115+080—at 5 GHz, achieving high angular resolution and sensitivity.
• Applied multi-epoch observations to detect variability in the radio emission, using time-domain analysis to distinguish intrinsic AGN variability from lensing effects.
• Compared observed radio luminosities with modelled dust emission components (from SED fitting) to assess whether radio emission exceeds expectations from star formation alone.
• Used smooth lens models (e.g., Oguri 2010) to predict flux ratios across multiple images and identify anomalies indicative of substructure or intrinsic variability.
• Assessed the position of the sources on the radio–FIR correlation to evaluate whether AGN activity could be heating dust and thus explaining the correlation without dominant star formation.
• Combined radio data with existing optical and infrared data to model the host galaxy and AGN contributions to the multiwavelength SED.

Experimental results

Research questions

• RQ1 Is the faint radio emission in radio-quiet quasars at z ≈ 1.7 primarily powered by AGN jets or star formation?
• RQ2 Can the radio–FIR correlation be used reliably to rule out AGN activity in RQQs with low radio luminosities?
• RQ3 Does intrinsic variability in the radio emission of lensed quasars mimic or mask microlensing or millilensing effects?
• RQ4 To what extent can host galaxy morphology (e.g., disk vs. spheroid) influence the observed radio emission in RQQs?
• RQ5 What constraints do flux ratio anomalies in lensed images place on the presence of dark matter substructures in the lensing galaxies?

Key findings

• In SDSS J1004+4112, strong evidence for AGN-related radio emission was found through significant variability in the radio light curve, particularly in component D, indicating intrinsic source variability rather than lensing effects.
• In PG 1115+080, the radio luminosity is consistent with the radio–FIR correlation, but the observed radio emission exceeds expectations from star formation alone, suggesting AGN-driven heating of dust may be responsible.
• The results in PG 1115+080 imply caution when using the radio–FIR correlation to infer star formation rates or rule out AGN activity in RQQs, especially in systems with low radio luminosities.
• The study confirms that two of the faintest radio sources ever imaged—both with intrinsic flux densities below 10 µJy—show strong evidence for AGN-dominated radio emission, challenging the assumption that such emission is primarily star-formation-driven.
• Flux ratio anomalies in PG 1115+080 were consistent with an extended source structure rather than dark matter substructure, and the anomaly in SDSS J1004+4112 at component C may be due to intrinsic variability, not microlensing.
• The findings support the idea that small-scale jets in RQQs may heat dust via shock fronts, placing the source on the radio–FIR correlation even without dominant star formation, thus implicating AGN feedback in shaping the host galaxy.