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 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 all four sources and resolve parsec-scale jets in two of them. The VLBI components show low brightness temperatures ( T b ≈ 10 9 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">R</mml:mi> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:mn>1.4</mml:mn> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">GHz</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Edd</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≈</mml:mo> <mml:mo>−</mml:mo> <mml:mn>5</mml:mn> </mml:math> 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 general relativistic magnetohydrodynamics 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.