Here Be SDRAGNs - Spiral Galaxies Hosting Large Double Radio Sources

Abstract

We present a sample of large double radio sources hosted by spiral galaxies (Spiral Double Radio Active Galactic Nuclei, SDRAGNs). Candidates were selected during Radio Galaxy Zoo, and refined using Sloan Digital Sky Survey images. The most promising were targeted in the Zoo Gems Hubble Space Telescope program, yielding images for 36 candidates. We assess the likelihood of each spiral galaxy being the genuine host of the radio emission finding 15 new high-probability SDRAGNs. SDRAGN hosts are seen preferentially close to edge-on. SDRAGNs predominantly show FR II radio structures and optical pseudobulges. Accounting for sample selection effects, the radio-jet axes lie preferentially near the poles of the galaxy disks; we find a constant probability distribution for intrinsic pole-jet angles < 30 degrees, ramping to zero at 60 degrees. We have obtained optical spectra for all these new SDRAGNs. Among both previous and new SDRAGN samples, 8/25 show Seyfert 2 signatures, 6/25 show central star formation, and 5/25 show LINER emission strong enough to indicate AGN or shock ionization, broadly similar to radio galaxies in elliptical hosts with the addition of star formation (diluting or masking weak AGN signatures). SDRAGNs include FR II sources seen at unusually low radio power, and preferentially occur in significant galaxy overdensities on 1-Mpc scales. Our "false alarms" - systems where HST data show the spiral to not be the actual host galaxy - include radio sources seen through large parts of foreground spiral disks, potentially useful for Faraday-rotation studies of disk magnetic fields.

Figures (14)

• Figure 1: Magnitude in $r$ versus SDSS axial ratio b/a for subgroups of the SDRAGN sample, illustrating selection effects with apparent magnitude. Small filled circles are galaxies in the sample from which the observing list was drawn but not selected for the HST target list, intermediate filled circles are galaxies in the HST target list but not observed, and large filled circles represent the objects observed and analyzed here. The eleven previously-identified SDRAGN systems are represented by plus signs. The brightest RGZ galaxies (12/14 of which are in the HST target sample) are seen systematically more face-on than a random set of SDSS disk systems, while the faintest ones are systematically more edge-on.
• Figure 2: Previously identified SDRAGN J2318+4314 (MCG +07-47-010), in $B$ and $R$ images from the Jacobus Kapteyn Telescope (JKT). Each is displayed with a logarithmic intensity mapping. North is at the top and east to the left; the field shown spans 160 E-W by 172 N-S.
• Figure 3: Montage of radio survey contours overlaid on SDSS $gri$ images for the fifteen probable SDRAGNs (with and without radio cores) from the RGZ-Zoo Gems sample. North is at the top and east to the left; horizontal scale bars indicate 5. Some double sources are so large that the galaxy cannot be well shown (and core contours are affected by smoothing at this scale); for these we include an enlarged "core" region as well. Radio data are from the VLA Sky Survey median of epochs 1--3 (green contours), and LoTSS (orange contours). Radio contours are spaced by factors of 2 in brightness, starting with the values in Jy per beam shown in the numbers in matching colors at the bottom of each panel. These represent values 1.8-2.5 times the RMS noise level, set to minimize appearance of interferometric artifacts, over ranges appropriate for showing important radio structures while reducing clutter. The SDSS images are shown in the Lupton "sinh" mapping to show detail over a wide dynamic range. Coordinate-based designations are as in Table \ref{['table-crossids']}.
• Figure 3: (continued)
• Figure 4: Low-resolution LoTSS image of J1322+2706 shown in grayscale to illustrate the radio structure. The left panel shows a negative linear intensity mapping, with 5-arcminute scale bar. The right panel shows the same mapping with a contour at the local $1-\sigma$ level of 0.125 mJy per beam, and a schematic interpretation of two elliptical, filamentary lobes seen in projection. The white cross shows the optical and radio nucleus; the size of the optical galaxy detected with HST (see Fig. \ref{['fig-HSTMontage']}) falls within the PSF of the radio nucleus here.