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 initially selected through the Radio Galaxy Zoo project and subsequently refined using Sloan Digital Sky Survey images. The most promising were targeted in the Zoo Gems Hubble Space Telescope (HST) program, yielding images for 36 candidates. We assess the likelihood that each spiral galaxy is the genuine host of the radio emission, finding 15 new high-probability SDRAGNs. The hosts are seen preferentially close to edge-on. SDRAGNs predominantly show type II Fanaroff–Riley (FR II) radio structures and optical pseudobulges. After accounting for sample selection effects, the radio-jet axes lie preferentially near the poles of the galactic disks; we find a constant probability distribution for intrinsic pole–jet angles ϕ < 30°, declining to zero at ϕ = 60°. We have obtained optical spectra for all these newly identified SDRAGNs. Among both previously known and new SDRAGN samples, 8/25 show Seyfert 2 signatures, 6/25 show central star formation, and 5/25 show low-ionization nuclear emission-line region emission strong enough to indicate active galactic nuclei (AGN) activity or shock ionization, broadly similar to radio galaxies in elliptical hosts but with the addition of star formation (diluting or masking weak AGN signatures). SDRAGNs include FR II sources seen at unusually low radio powers, and preferentially occur in significant galaxy overdensities on 1 Mpc scales. Our “false alarms”—systems where HST data show the spiral is not the actual host galaxy—include radio sources seen through large portions of foreground spiral disks, potentially providing useful probes for Faraday rotation studies of disk magnetic fields.