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SYFY WIRE Bad Astronomy

One ring (galaxy) to bring them all, and in the darkness bind them

By Phil Plait
Hoag’s Object is a perfect example of a ring galaxy, about 600 million light years away. Credit: NASA and The Hubble Heritage Team (STScI/AURA); Acknowledgment: Ray A. Lucas (STScI/AURA)

It's the kind of galaxy Sauron would love.

You've probably seen images of spiral galaxies, and elliptical galaxies, and even weird, distorted irregular galaxies. But have you ever seen a ring galaxy?

These probably started out as disk galaxies, much like our Milky Way. But then they suffered a direct hit collision with a smaller galaxy, the latter punching right through the middle of the bigger galaxy. The changing gravitational influence of the smaller galaxy as it approaches and leaves creates an expanding ripple in the bigger galaxy, what astronomers call a radially propagating density wave. It sweeps up gas and compresses it, forming an expanding ring of star formation.

The Cartwheel Galaxy, a relatively close by ring galaxy 500 million light years away. Credit: NASA/ESA/Hubble Borne, processed by Judy Schmidt

They're rare now, happening in only about 0.1% of disk galaxies in the local Universe. Due to a higher collision rate in the past, it's expected there would be more 10 billion years ago than there are today. But in fact they're rare back then, too. Searches for them haven't turned up many.

But a new deep survey of the sky yielded several, including one called R5519, which turns out to be big, bright, and fairly weird.

Hubble observations of the extremely distant ring galaxy R5519 show it in visible and near-infrared light (left), infrared (middle, with additional image from the Magellan telescope), and also in ultraviolet and visible light (right) which is an indicator

It's a whopping 10.7 billion light years away, so we're seeing it as it was when the Universe was only 3 billion years old. That right away is interesting; disk galaxies were rare at that time so seeing one this far away is pretty amazing.

It's part of a small group of galaxies, and in fact one of them, R5593, may be the culprit bullet galaxy that plunged through the heart of R5519; it has two nuclei (maybe it's merging with another small galaxy) and a protuberance pointing away from the ring galaxy. That could be due to the influence of the gravity of the ring galaxy on R5593, pulling out a streamer of gas and stars in what's called a tidal tail. That's common during collisions. R5593 is something like 100,000 light years from the ring galaxy now, indicating the collision was something like 40 million years ago.

The ring is over 30,000 light years across, and is furiously making stars at a rate at least 50 times what the Milky Way does. That's different than the ring galaxies that are nearer to us in the local Universe, which tend to have bigger rings but make stars at a much lower rate.

R5519 is also surrounded by a large diffuse disk of stars that's something like twice the size of the Milky Way's disk, and far larger than most other galaxies that are 10 billion light years from us. It's unclear why the ring galaxy would be so different from ring galaxies closer to us, and from galaxies local to it. It really does stand out as an oddball.

Hoag’s Object is a perfect example of a ring galaxy, about 600 million light years away. Credit: NASA and The Hubble Heritage Team (STScI/AURA); Acknowledgment: Ray A. Lucas (STScI/AURA)

The collisions that form ring galaxies commonly make two rings, one forming as the bullet galaxy approaches, and the second as it recedes. It's not clear if the ring in R5519 is the first or second one; different methods for determining its age yield results consistent with either. Perhaps further work can help narrow down it age, which in turn will help astronomers understand the mechanics of the collision and its aftermath.

The importance of this galaxy is that it tells us about conditions for galaxy formation and evolution when the Universe was much younger. Given that collisions were more frequent back then, why are there so few ring galaxies? The astronomers who observed R5519 posit that there were fewer disk galaxies at the time, and it's also possible there were so many collisions that ring structures were destroyed quickly too. There weren't many small galaxy groups either, and the dynamics of these groups may aid in the formation of rings.

Not much is known about ring galaxies more than about a billion light years away from us. This one is much farther than that, so perhaps could help anchor some ideas about how the formation of these bizarre objects changes over time. We're still trying to figure out how disk galaxies form in the first place, and strange examples help show the extremes of what can occur.

R5519 may not be the One Ring to Rule Them All (for ring galaxies I'd make that call for Hoag's Object, pictured above), but given how it may help us understand not just ring galaxy formation but also disk galaxy formation as well — like the one we happen to live in — then it really is the "one ring to bring them all, and in the darkness bind them."