Put a ring on it

The intergalactic environment is a messy place. When you've got masses of hundreds of billions of Suns hurtling past each other at hundreds of miles a second, you expect things to get ugly. What you might not expect is to find nice, neat rings.

To be fair, rings are pretty rare structures. Ring galaxies like the famous Cartwheel have been shown to (most likely) form during collisions of very specific encounter geometries. Others, like the implausibly neat Hoag's Object, are more mysterious.

Ring galaxies typically have rings with highly active star formation and lots of gas. But there are a very few gas rings without any appreciable star formation activity at all. The most famous is the giant Leo Ring :

This is also thought to have been formed during an encounter. Of course there's also the much smaller but weirder Keenan's Ring :

Which is especially strange because the Ring is strongly offset from the main galaxy in this region and doesn't show much of a velocity gradient across it. That makes it very hard to explain by a collision.

Today's paper announces the discovery of another spectacular ring quite similar to the Leo giant. It's about half the size, but still much larger than Keenan's Ring. It's got about 3 billion solar masses of gas but not much in the way of associated stars. Actually, they point out a few stellar smudges within the structure that could be part of the Ring but it's by no means clear this is actually the case, so it might be completely optically dark. What's especially strange about this one is that the central galaxy is an elliptical, which don't usually have much gas at all.

Like Keenan's Ring, this feature is distinctly offset from the central galaxy - although not nearly as much as in the case of Keenan's. On the other hand, it has a much larger velocity gradient, making a collisional origin somewhat more likely. The lack of any stellar disturbances is a bit odd though.

What could be going on ? Well, in elliptical galaxies which do have gas, it's thought the gas density is much lower than in spiral galaxies. So the authors suggest a collision in those cases would have quite different effects to more spectacular features like the Cartwheel : the compression of the gas just isn't enough for it to reach the densities needed for star formation. Additionally, the shock heating from the compression may be more effective, making most of the gas ionised and unable to form stars. Alternatively, the ring could be the remains of a disrupted gas-rich galaxy that fell into an unpleasant orbit around the elliptical and never escaped.

There's not much more we can say at this stage other than, "wow, a giant ring, cool !". Observations of ionised gas could help trace other components and see if this really was formed by collision, while simulations could show if there's a plausible encounter that really could form a gas ring that leaves the stars so completely undisturbed. We'll just have to wait and see.

Discovery of a large HI ring around the quiescent galaxy AGC 203001 

Here we report the discovery with the Giant Metrewave Radio Telescope of an extremely large ($\sim$115 kpc in diameter) HI ring off-centered from a massive quenched galaxy, AGC 203001. This ring does not have any bright extended optical counterpart, unlike several other known ring galaxies.