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 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 ($β\lesssim 0.6$ for inclinations $θ\lesssim 80^\circ$), 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.

Figures (4)

• Figure 1: Optical and radio images of the four targets at various spatial scales. HST (SDSS) optical images are shown for J0219+0155 and J1352+3126 (J1159+5820 and J1649+2635). As for radio, LOFAR (VLA) images are shown for J1159+5820 and J1352+3126 (J0219+0155 and J1649+2635). All the VLBI images are obtained by the VLBA. The left and center columns show kpc-scale radio (red contours) and optical (color, in $\sqrt{\rm intensity}$) images of the targets in absolute RA and Dec. The right column shows the parsec-scale VLBI images, showing the total intensities in both contours and color in relative RA and Dec. The corresponding VLBI beam sizes are shown as green ellipses in the bottom right corner of each panel. In each row, the source names are shown in the top left corner of the leftmost image. In all panels but kpc-scale radio structures of J1159+5820 and J1352+3126, contours are displayed at $(-1, 1, 2, 4, 8, 16, 32, 64, 128, 256) \times 3\sigma$ levels. For kpc-scale radio images of J1159+5820 and J1352+3126, higher starting levels of $4\sigma$ and $6\sigma$ are adopted, respectively. Bars at the bottom left corners of each panel show the corresponding spatial scales.
• Figure 2: Distribution of the Gaussian components fitted to the VLBA data. In all panels, contours show the total intensities and blue circles with crosses indicate the center locations FWHM sizes of the Gaussians with their corresponding IDs (see Table \ref{['tab:BT']}). Green ellipses in the bottom right are the corresponding VLBA beams. The source names are indicated on the upper left corners. Top right and middle left panels are zoomed-in crops of the top left panels. In all panels, the contours correspond to ($-$1, 1, 2, 4, 8, 16, 32, 64, 128, 256)$\times 3\sigma$ of the individual images.
• Figure 3: Possible jet speeds of (upper) J1159+5820 and (lower) J1352+3126, derived based on the scenarios outlined in the main text and Sect. \ref{['sec:structure_identification']}. In both panels, the black solid line and the gray regions represent best-fit and allowed ranges of the jet speeds including uncertainties in $R_{\rm jet}$ ($1\sigma$ level), respectively
• Figure 4: The fundamental plane of active black holes Merloni03, including data from Merloni03 and those of three of four targets in our study. The latter is highlighted by error bars and colored dashed lines with corresponding source names. Upper limits are marked with arrows and shown in empty symbols. The color denotes ranges of $M_{\rm BH}$. Different symbols show types of the BHs---'GBH': Galactic black holes, 'Sy': Seyfert galaxies, 'L': Low-ionization nuclear emission-line regions, 'T': Transition objects (Liner / H II), 'QSO': Quasi-stellar objects.