VLBI Detections of Compact Nuclei in Spiral-hosted Double-lobed Radio-loud Active Galactic Nuclei (DRAGNs): Evidence for Weak Parsec-Scale Jet Activity

TL;DR

This study uses VLBI observations to detect and analyze weak, parsec-scale jets in spiral-hosted double-lobed radio-loud AGNs, challenging traditional views and suggesting recurrent jet activity in such galaxies.

Contribution

It provides the first VLBI detections of compact nuclei in spiral DRAGNs and links their jet activity to accretion processes, supported by observational data and simulations.

Findings

Detected compact cores in four spiral DRAGNs.

Identified mildly relativistic, weakly powered jets.

Evidence of recurrent or long-duty-cycle jet activity.

Abstract

We report milliarcsecond-scale VLBI detections of compact radio nuclei in four spiral-hosted, double-lobed radio-loud AGNs (spiral DRAGNs), a rare class that challenges the traditional association of powerful jets with elliptical hosts. Using public VLBI data archives, we identify compact cores in four sources and resolve parsec-scale jets in two of them. The VLBI components show low brightness temperatures ($T_{\rm b} \approx 10^9$ K in the core) and jet-to-counterjet ratios consistent with only mildly relativistic intrinsic speeds ($\beta \lesssim 0.6$ for inclinations $\theta \lesssim 80^\circ$), indicating weakly powered pc-scale outflows. The low radio-Eddington ratios $\log(L_{\rm R,1.4\,GHz}/L_{\rm Edd}) \approx -5$ to $-8$ support this interpretation. Three objects lie on the fundamental plane of black hole activity, implying that global accretion-jet coupling in spiral DRAGNs is similar to that in other AGNs. Comparison with recent GRMHD simulations of thin-disk jets suggests that the VLBI-scale cores in spiral DRAGNs may trace an early or intermittently magnetized phase of jet launching. The coexistence of weak pc-scale jets and large kpc-scale lobes implies recurrent or long-duty-cycle jet activity in these late-type hosts.