VLA FRAMEx. III. Circumnuclear Radio Emission Mechanisms in Hard X-ray Selected Active Galactic Nuclei

TL;DR

The paper addresses the origin of circumnuclear radio emission in hard X-ray selected AGNs by combining uniform VLA 4–12 GHz imaging with prior VLBA and X-ray data. It tests coronal, jet/bubble, wind, and star-formation scenarios, using size estimates, energy budgets, and multiwavelength diagnostics to disentangle emission mechanisms at tens of parsecs. The key finding is that extranuclear radio emission in most FRAMEx targets is consistent with AGN winds interacting with the ISM, with coronal emission plausible only for a subset detected at subparsec scales, and star formation contributing but not driving the majority. The work highlights the need for intermediate-scale observations (e.g., e-MERLIN/ngVLA) and coordinated multiwavelength modeling to fully constrain the physical origins of circumnuclear radio emission in nearby AGNs.

Abstract

We present Stokes I continuum analysis for a volume-limited sample ($<40~\rm{Mpc}$) of hard X-ray selected active galactic nuclei (AGNs) using $4-12$ GHz observations with the Karl G. Jansky Very Large Array (VLA). All of the 25 sources analyzed here have previously been observed with the Very Long Baseline Array (VLBA) to probe their subparsec projected physical scales, but detected emission has only been measured for 12 of the sources at $C$ band (4.4 GHz), despite expectations. We determined that coronal emission is unlikely to be a dominant emission mechanism for the sources not detected by the VLBA, and the emission measured with the VLA is likely produced beyond parsec spatial scales. We also explore potential radiation mechanisms for the circumnuclear radio emission that is produced beyond the observable ~parsec physical scales probed with the VLBA but within the $\leq30-110$ parsec spatial scales observed with the VLA. From an energetics perspective, we find that all targets have extranuclear radio emission that is compatible with AGN winds, assuming a maximum of 10% of the bolometric output can supply the mechanical energy observed. We also find that the excess emission is likely too strong for star formation alone when compared to results from optical spectroscopy, but may contribute in smaller capacities.

Figures (3)

• Figure 1: \ref{['fig:lrlx_vla']} Radio luminosities from VLA observations 2025ApJ...986..194S versus X-ray luminosities [][and this work]2021ApJ...906...88F. Solid black circles represent VLBA-detected objects and solid red squares represent VLBA non-detections. Hollow grey circles represent optically selected AGNs 2008MNRAS.390..847L. The dotted green line represents the Güdel-Benz relation for coronally active stars, and the dashed-dotted line represents the radio-loud AGN threshold defined by 2007AA...467..519P. \ref{['fig:lrlx_vlba']} Same as Figure \ref{['fig:lrlx_vla']} but radio luminosities are determined from the VLBA measurements with upper limits indicated for VLBA non-detections ($5\times{\rm rms}$ at 6 GHz; see Table \ref{['tab:framex']}).
• Figure 2: \ref{['fig:winds']} Distribution of wind energetics. Orange indicates the circumnuclear radio energies attributed to wind interactions as a percentage of the minimal wind outflow ($\frac{\rm KE_{max}}{\rm Rad._{min}}$). The blue indicates the circumnuclear radio energies attributed to wind interactions as a percentage of the maximal wind outflow ($\frac{\rm KE_{max}}{\rm Rad._{max}}$). \ref{['fig:sfrs']} Excess radio SFRs versus narrow-line H$\alpha$ SFRs. Red are the VLBA non-detected FRAMEx sources and black are the VLBA detected sources.
• Figure 3: Excess VLA emission as a function of VLBA source brightness with upper limits indicated for VLBA non-detections (see Table \ref{['tab:framex']}). The dotted line indicates a one-to-one ratio.