Investigating the origin of radio emission in candidate super-Eddington accreting black holes

TL;DR

This study probes the origin of radio emission in 18 candidate super-Eddington xA quasars by combining new VLA data with LOFAR and VLASS and integrating optical/IR measurements. A multi-indicator framework (variability, luminosity, morphology, spectral index, and FIR–radio checks) reveals that SF-related processes dominate or strongly contribute to the radio output in most sources, with only a minority showing core/jet–driven emission; three cases even suggest concurrent super-Eddington accretion and relativistic ejections. The results support a picture where high accretion rate AGN exhibit substantial SF-driven or wind-related radio structures rather than prominent relativistic jets, consistent with a common evolutionary path on the quasar main sequence. These findings have implications for understanding feedback mechanisms and the co-evolution of black holes and their host galaxies, particularly in the early growth phases of super-Eddington systems, and highlight the value of multi-wavelength studies for disentangling radio emission origins. The work also identifies a few intriguing jet-bearing, high-$L/L_ ext{Edd}$ candidates suitable for targeted follow-up with next-generation facilities like the Square Kilometre Array.

Abstract

Recent works show that the radio power of quasars accreting at very high rates can reach surprisingly high values. These studies suggest that this radio emission might originate from star formation, but lack of data leaves open the possibility that they could also contain a jetted active galactic nucleus (AGN). We investigate the origin of the radio emission of a sample of 18 super-Eddington candidates, over a wide range of redshifts. These sources are expected to have extreme radiative output per unit black hole mass, show high-velocity outflows and are therefore thought to be a prime mover of galactic evolution via radiative and mechanical feedback. We present new Karl G. Jansky Very Large Array (VLA) observations at L, C and X-band of these sources, which we combine with observations from the LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) and the Very Large Array Sky Survey (VLASS). We also use optical and IR data to derive estimates of accretion and wind parameters, as well as star formation rates to compare with the ones derived from the radio emission. Based on the radio variability, luminosity, morphology, radio spectral properties, radio vs IR estimates of star formation rate and radio-to-mid IR flux ratio, we find that 7 of our 18 targets are likely to have their radio emission predominantly coming from SF, and 6 from a combination of SF and AGN-related mechanisms, while only three sources indicate a core or jetted AGN only origin for the detected radio emission. This is consistent with previous studies, and supports the prevalence of lower power radio structures associated with star-forming activity rather than relativistic jets in the high Eddington ratio regime. In the same sample, however, we find three sources for which the data suggest a concomitant presence of super-Eddington accretion and relativistic ejections.

Figures (18)

• Figure 1: Prevalence of FIRST-detected type-1 AGN marziani_is_2013 (blue circles and cyan shading) and median radio power (red circles and rose shading) as a function of spectral type along the quasar main sequence.
• Figure 2: Radio morphology distribution across the redshifts of the sources. C indicates compact at all frequencies observed, I is intermediate morphology detected in at least one band, and E indicates the presence of diffuse emission in at least one band.
• Figure 3: The spectral indices $\alpha_{1.5-5}$ versus $\alpha_{0.144-1.5}$, $\alpha_{3-5}$ versus $\alpha_{1.5-3}$, $\alpha_{5-10}$ versus $\alpha_{3-5}$ and $\alpha_{5-10}$ versus $\alpha_{1.5-5}$. Markers are colored depending on the identified dominating mechanism behind the detected radio emission (Table \ref{['table_classification']}). The dashed line is the $1:1$ ratio line of equal slopes, while the horizontal and vertical dotted lines are $\alpha = -0.5$. The area showing flat or inverted spectra, which may be associated with a core-dominated jet or a corona, is filled in gray. The area with a spectral index $\alpha = -0.8 \pm 0.4$ with a scatter of 0.2, which may be related to SF, is filled in turquoise. The area showing very steep spectral slopes with $\alpha_{5-10} < -2$ in the right-hand panel, which may imply possible relic emission, is filled in blue.
• Figure 4: Distribution of the two-point spectral index between radio (1.4 GHz) and mid-IR ($22\mu m$). All sources have $\alpha^{22}_{1.5}$ below $\sim-0.1$ except J12091+5611 and J14258+3946.
• Figure 5: SFR estimate based on FIR luminosity vs. radio pseudo-SFR for all sources radio-detected and with IRAS $60\mu m$ data (black diamonds). Star-forming galaxies (green circles), RQ quasars (blue circles) and RL quasars (pink circles) are from bonzini_star_2015, with their $p\text{SFR}_{\text{radio}}$ converted using eq. 22 of molnar_non-linear_2021. The dashed line is the 1 : 1 ratio line of equal SFR.