... you put a ring on it, obviously.
Sorry.
Anyway, today's paper is about one of the rarest sorts of galaxy morphology, the polar ring – that is, a ring orbiting at a high angle over the plane of the galaxy disc. If you've ever seen my introductory galaxy evolution course, you know that rings are very much a thing, but what I didn't mention was just how rare they are. Which is very. According to the authors, previous estimates put them at a frequency as low as about one in a thousand.
Being incredibly rare either makes them automatically interesting or just unimportant, depending on your point of view. In the memorable words of Alan Dressler, polar ring galaxies "look odd and forced, like a dog wearing shoes". Personally, I would lean towards the discovery of any dog that voluntarily decided to wear shoes as being super interesting, even if learning all about such an animal wouldn't tell us much about the general doggy population.
How do they form ? Two main mechanisms : direct head-on collisions of two galaxies, and the result of warps in galactic discs. If it's the latter, they might just be extreme cases of warps that result from natural instabilities from the rotation. Either way they wouldn't last very long, but could tell us more about how galaxies evolve when left to their own devices or when they interact. It could also be that a much higher fraction of galaxies experience a ring at some point, and that we only see a very small fraction with rings because no individual structure lasts very long.
Polar rings are generally seen in stellar emission. The authors here found a couple of candidates using an HI survey, with the stellar component of one of the rings being very faint and not detected at all in the other. But these are two galaxies out of just two hundred in their sample, suggesting that maybe gaseous rings are an order of magnitude more common than stellar rings. Of course with such small sample sizes this is reaching a bit, but it's still an intriguing early result from the shiny new WALLABY survey.
Sadly they don't really speculate much on the origin of the rings in this case. The environment of the galaxies is barely mentioned, since their main concern is to just to establish that they are rings. Personally I think they overdo it; to me I'd be inclined to say, "yep, that's a ring", but they really go to town on this. They make model polar/inclined rings and transform them into mock observations to see how much the noise and beam shape would affect their detectability, with the end result that one candidate is more likely a PRG than not, and the other is a strong possibility (the major alternative being that it's just a warp in the disc) but needs better resolution.
They also note that the appearance of the rings changes depending on resolution : at lower resolution, they're easier to see by looking at kinematic maps, whereas at higher resolution ordinary integrated flux maps show them more clearly. Once again data visualisation matters. And it's nice to see that they actually do a lot of their measurements using the virtual reality iDaVIE software; it deserves some serious use rather than being an entertaining gimmick.
All this means that we might be missing a significant number of ring galaxies, both from looking at the wrong wavelengths and making the wrong sort of maps. It's unlikely that rings are so numerous as to constitute a significant portion of the galaxy population, but in terms of understanding the rings themselves, this could be really important. One bit of speculation that they do indulge in is to note that the HI rings might be the progenitors of the stellar rings, or perhaps the two form by completely different mechanisms.
Predictably this ends in a classic case of "we need more data". But that's okay, because they establish good ways to identify rings and WALLABY will in fact provide that data. With WALLABY alone expecting to increase the number of HI detections into the hundreds of thousands, it's going to be interesting to see how this develops over the next few years.
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