D-28: The Puzzle

The following was originally published on the Stellaris forums.


We’re back! Good morning. Today we’re going to talk about straight-up nonsense which cannot possibly exist and which violates everything we know about galaxies.

Please meet Hoag’s Object.
(Image courtesy Cornell.)

I’m not saying that it was definitely formed by witches. As a scientist I can’t say that, since we haven’t observed clear evidence of their activity and since other possibilities exist. (Necromancers, for example.) However, my suspicion is that it was witches.

If you’re not an astrophysicist, you may not understand why I am freaking out this badly over this galaxy. Let me explain.

Most galaxies are either elliptical or spiral (or lenticular, but they aren’t important here.) Elliptical galaxies don’t have much dust or gas in their halo, and are composed of mostly old stars blobbed together without much structure. Spiral galaxies have large amounts of dust and gas which leads to many new, young stars. They have beautiful and elegant “arm” structures which reach out from the core.

Hoag’s Object has a bright yellow core which is composed of older stars, like an elliptical galaxy. However it also has a ring of young stars surrounded by a huge disc of gas, like a spiral galaxy.

So is it a combination of the two? No. The older core is almost perfectly spherical, which is basically unknown in elliptical galaxies; and the outside is ring-shaped instead of arm-shaped, which… look at it! Just look at it! That doesn’t happen by accident. Something is up here.

Witch-hysteria aside, how could it have formed?

Ring-shaped galaxies aren’t entirely unknown. (If you look hard at the picture you can actually see another of them through the gap in Hoag’s Object; it appears red in that picture.) In most cases they occur because of collisions between galaxies. If a smaller galaxy acted like a “bullet” and plowed into the middle of a spiral galaxy at high speed, it would push the stars outwards in a huge but very slow wave, and leave only an outer ring separated from what’s left of the core.

We don’t think this is what happened with Hoag’s Object. For one thing, the core stars of Hoag’s Object are much older than the stars in the ring. For another thing, the cores of spiral galaxies aren’t perfectly spherical like Hoag’s Object’s core is.

There’s also the small matter that we can’t spot anything that could have been the “bullet.” Tracking the motion of galaxies through space isn’t that hard – they’re not small and they tend to move in steady directions – but there’s nothing we can find. It couldn’t just have disappeared.

Arthur Hoag, its discoverer and namesake, suggested that Hoag’s Object was actually an optical illusion caused by the “gravitational lensing” of a massive black hole. This was plausible when he suggested it in 1950 and would be a neat solution but we’ve since confirmed that it isn’t the case.

Noah Brosch suggested another method: it could have been a bar galaxy (a type of spiral galaxy that’s growing more common as the galaxy ages) which had its centre fall apart and had the spirals drift outwards into a ring. However, this would also result in a disc-shaped core rather than the spherical core we see here, and even Brosch has admitted that this is unlikely given what we know of bar galaxies.

Ultimately we just don’t know.

Don’t leave me in suspense here! Was it witches, or indeed something less silly like aliens?

I’m not saying that it was witches. I’m also not saying that it wasn’t. All I’m saying is, keep a bucket of water handy if you go there.

All silliness aside, Hoag’s Object leaves us with exactly two possibilities: We either know a lot less about galaxy formation than we thought we did, or there’s something out there which can shift stars around in ways we couldn’t even dream of. Neither is a nice thought.


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