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

Abstract

We report milliarcsecond-scale VLBI detections of compact radio nuclei in four spiral-hosted, doublelobed 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 (Tb ≈ 109 K in the core) and jet-to-counterjet ratios consistent with only mildly relativistic intrinsic speeds (β ≲ 0.6 for inclinations θ ≲ 80◦ ), indicating weakly powered pc-scale outflows. The low radio-Eddington ratios log(LR,1.4 GHz/LEdd) ≈ −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.