Here be SDRAGNS! Results from Radio Galaxy Zoo and Hubble’s Zoo Gems

It’s taken a while to get this finished, but I am happy to say that our paper combining Radio Galaxy Zoo and Hubble data on the rare spiral galaxies with large double radio sources (also known as SDRAGNs, Spiral Double Radio AGNs) has been accepted by the Astronomical Journal. The RGZ-HST sample is the largest set of such objects known (we found 15 cases, compared to 11 from everyone else published up through late 2025). With the collaboration of Alexei Moiseev and students using the 6-meter telescope, we could complete the set of redshifts and optical spectroscopic properties for these SDRAGNs

The Zoo Gems project of short-exposure Hubble observations gave us images of 36 potential SDRAGNs. Most of these turned out to be something else – a disturbed but not spiral galaxy, a spiral almost in front of the more distant radio galaxy… Still, we confirmed enough to more than double the known sample of these rare systems from 11 to 26, selected in more systematic ways than their predecessors. As a group, SDRAGNs have a wide range of Hubble types, from Sa to Sc – this was a bit unexpected, since the mass of the central black holes correlates with the bulge starlight, and the radio sources are probably powered by very massive black holes. (Also, another group including Wu, Ho, and Zhuang analyzed many of the Zoo Gems SDRAGN candidates and found that most of them have pseudobulges rather than classical bulges, which suggests that these galaxies have not undergone a major merger over their history). SDRAGN host galaxies are seen nearly edge-on more often than would be expected for a random set of spirals. We do not see many strong interactions, although there are several SDRAGNs with dust lanes twisted out of the galaxy plane which could result from a weak interaction a billion years before our current view. These galaxies occur in denser environments than average as traced by other galaxies, which fits with our understanding of the need to have circumgalactic gas for the radio jets to interact with in order to produce the powerful lobes of radio-emitting material. Combining the radio structures with galaxy properties from the Hubble images, perhaps our key results is that the radio jets merge preferentially within about 30 degrees of the poles of the galaxy disks. This helps understand why the jets make it outside the galaxy – they encounter the least interference from gas within the galaxy that way. This contrasts with the random orientations of those radio jets which happen in spiral Seyfert galaxies, which really do seem to have random directions and mostly dissolve within a few thousand light-years as they encounter the dense gas within the galaxy itself. Returning to the incidence of pseudobulges, within a major merger we expect the black hole to grow by incorporating material from within the galaxy, so it would keep roughly the same “spin” direction as the galaxy disk and impart that to the accretion disk and jets. This directionality seems to be more important than the mass or accretion rate of the black hole in producing SDRAGNs.

As examples of the data we could work with, here is a montage of 9 SDRAGNs using SDSS images overlaid with VLA Sky Survey contours (green) and contours from the lower-frequency LOFAR sky survey (orange). The LOFAR data became available only after the original Radio Galaxy, and are much more sensitive to the diffuse emission from radio lobes.

This montage shows the same nine SDRAGNs in negative views of the Hubble blue-light images. They are oriented so the galaxy plane is horizontal; arrows mark north and are 5 arcseconds long (matching the scale bars on the radio overlays).

Given the roots of Radio Galaxy Zoo and Zoo Gems in the Galaxy Zoo family, it would happen that one of the Hubble targets which turns out to not be a spiral does have a giant emission region similar to Voorwerpjes. We can’t help it, they are everywhere (and from some JWST data, almost everywhen too).

The manuscript is available from the arxiv repository (and from the AJ web site in a couple of months, after formal publication). Much of the text in the first two sections comes from a draft written by the late Jean Tate even before Hubble data started to arrive, and once again we regret that he did not live to see some of the later Hubble images. This was a favorite project of Jean, who managed most of the initial voting to select HST imaging candidates and kept the SDRAGN material in a PBWorks online repository, so well organized that we could reconstruct a great deal of the project detail from there. (The paper includes the master table of all 215 RGZ SDRAGN candidates in case someone else wants to follow them up).