The Quasar Feedback Survey: Revealing the importance of sensitive radio imaging for AGN identification deeper into the radio-quiet regime

TL;DR

This work extends the Quasar Feedback Survey into the radio-quiet regime with 29 new z<0.2 quasars imaged at sub-arcsecond resolution in L- and C-bands. It reveals that a substantial fraction host AGN-driven radio emission—through compact jets and jet-like outflows—despite lacking classical radio-loud classifications, and most sources exhibit steep spectra indicative of optically thin synchrotron processes. By combining morphology, spectral index, and brightness temperature diagnostics, the study identifies ~38% of QFeedS-2 as radio-AGN (a lower limit given data limitations), and, when combined with the prior QFeedS-1, indicates that ~66% of the full sample shows radio-AGN activity across nearly four orders of magnitude in radio power. The results imply that compact, low-power jets and AGN shocks are a common feedback mode deep into the radio-quiet regime, underscoring the need for high-resolution, multi-wavelength campaigns to connect radio properties with multi-phase outflows and galaxy evolution.

Abstract

We present new sub-arcsecond ($\sim$0.3-1 arcsec; $\sim$1--3\,kpc) VLA imaging at 1.4\,GHz and 6\,GHz of 29 optically-selected, [O~{\sc iii}] luminous ($L_{\rm [O III]}$ > 10$^{42.1}$\,erg\,s$^{-1}$), $z<0.2$ quasars drawn from the expanded Quasar Feedback Survey (QFeedS; with $L_\mathrm{1.4\,GHz} = 10^{22.6}$--10$^{26.3}$\,W\,Hz$^{-1}$). These 29 new objects occupy the low end of the radio-power distribution ($L_\mathrm{1.4\,GHz}$=$10^{22.63}$--10$^{23.45}$\,W\,Hz$^{-1}$) in the QFeedS sample and are nominally `radio quiet'. Despite this, we find widespread evidence of AGN-driven synchrotron activity. Nearly $\sim 31\,$per\,cent exhibit resolved radio structures on $\sim$0.1--20\,kpc scales consistent with compact jets or wind-driven outflows, and $\sim 90\,$per\,cent display steep spectra ($α\lesssim -1$) indicative of optically thin synchrotron emission. Combining morphology, spectral index and brightness-temperature diagnostics, at least $\sim38\,$per\,cent of the sample show clear AGN signatures that cannot be explained by star formation alone. These constitute the first results from the expanded QFeedS (now 71 quasars spanning $\approx 4$ dex in radio power) and demonstrate that compact, low-power jets and AGN shocks are common deep inside the radio-quiet regime. A thorough understanding of feedback processes from quasars, deep into the `radio-quiet' regime, will be obtained by connecting these high resolution radio observations with multi-wavelength observations.

Figures (7)

• Figure 1: Top: 1.4 GHz radio luminosity versus [O III] luminosity for the parent sample of $z<0.2$ AGN (grey circles, with radio upper limits represented as lighter grey triangles; Mullaney2013). The dashed vertical line shows our quasar luminosity threshold for selection ($L_{\rm [O III]}>10^{42.11}$ erg s$^{-1}$). Bottom: [O iii] FWHM versus 1.4 GHz radio luminosity for the parent sample above the [O III] luminosity cut (grey dots, with upper limits represented as arrows). In both panels the initial 42 QFeedS targets are represented with black stars (QFeedS-1), for which we imposed a minimum radio luminosity threshold ($L_{\rm 1.4GHz}>10^{23.45}$ W Hz$^{-1}$; dashed horizontal line in top panel). The 29 new targets, which are selected to be below this radio luminosity (QFeedS-2), are represented with green stars. Type 1 quasars are highlighted with white circles in the bottom panel.
• Figure 2: Examples of our radio data for sources classified as compact based on radio morphology in the QFeedS-2 sample (see Section \ref{['sec:radio_morph']}). The rgb images come from the DESI Legacy Imaging Survey in the $z,r,g$ bands. The contours overlaid represent: FIRST maps in white with levels $1\sigma \times [2,3,4,5]$; our new VLA 1.4 GHz maps in green with levels $1\sigma \times [2,4,6,8]$ (except for J0213$+$0042, which was not observed) ; and our new VLA 6 GHz maps in blue with levels $1\sigma \times [2,4,6,8]$. Ellipses represent the synthesised beam for the corresponding radio map contours; $\sim$5 arcsec for FIRST (in dashed grey ellipses), $\sim$1 arcsec for VLA 1.4 GHz (in green) and $\sim$0.3 arcsec for VLA 6 GHz ( in light blue). The scale bar highlights the physical size scales. Equivalent figures for all sources are provided (see the Data Availability section). For J0213+0042, we note the target was classified as a 'compact' source because the second radio component, which coincides with a second optical companion, was ignored.
• Figure 3: The same as Figure \ref{['fig:compact_sources']}, but presenting the nine targets identified as having extended 'jet-like' structures. These targets are classified as radio-AGN based on radio morphology. We note that the target J0232$-$0811 was not observed in the L-band.
• Figure 4: Radio size versus radio luminosity for the QFeedS quasars compared to classical radio-selected AGN populations. Coloured points and contours mark the loci of GPS, CSS, FRI nad FRII sources, while the shaded yellow region highlights the FR0 domain as defined in Baldi2023. Black stars show the QFeedS-1 sample Jarvis2021, and green stars show the new QFeedS-2 sample presented in this work. The dashed line marks the QFeedS-1 radio luminosity selection limit. The majority of QFeedS-2 remain compact at sub-arcsecond to arcsecond scales ($\sim3\,$kpc), and broadly consistent with FR0-like systems. This distribution closely parallels the compact steep-spectrum sources reported in Chilufya2024 (plus symbols), though QFeedS quasars are optically selected and radiatively efficient, in contrast to many radio-selected samples. By contrast, QFeedS-1 shows a higher fraction of extended sources with higher radio luminosities, bridging towards CSS and FRI-like populations. At the same time, the QFeedS-2 sample overlaps in luminosity with the intermediate-power HERGs of Pierce2020 (cross symbols), but stands out for its strong [O iii] emission despite compact radio morphologies. Taken together, QFeedS maps a broad continuum of radio properties in optically-selected quasars and highlights the role of QFeedS in connecting low-luminosity FR0-like AGN with the more extended radio populations, while demonstrating the diversity of radio output among radiatively-efficient quasars across several orders of magnitude in $L_{1.4\,\mathrm{GHz}}$.
• Figure 5: Brightness temperature ($T_\mathrm{B}$) as a function of spectral index, $\alpha$, ($\sim$5--7 GHz) for the whole Quasar Feedback Survey sample. Black stars highlight the QFeedS-1 sample of 42 targets as presented in Jarvis2021. The new sample of 29 targets presented in this work (QFeedS-2) is highlighted by green stars. For the multi-component sources, we plotted the component with the highest $T_\mathrm{B}$ value. The dashed red line highlights the lower limit of brightness temperature $T_\mathrm{B} = 10^{4.6}\,$K for selected radio AGN. The vertical dot-dashed line separates flat and steep sources. For sources in the bottom-left quadrant, these radio measurements can not distinguish between AGN and star formation (SF). However, the targets highlighted by green squares indicate the QFeedS-2 sources that were classified as radio-AGN based on their morphology (Section \ref{['sec:radio_morph']}).