The Milky Way — our home galaxy — is a gorgeous spiral with a disk some 120,000 light-across, putting it in the top percentiles of disk galaxies in terms of size. We’re not sure exactly how it grew to such large proportions, though it certainly ate a lot of smaller galaxies in the process.

Recently, astronomers were surprised to see disk galaxies with well-defined central bulges of stars and obvious spiral arms even in the early Universe, before it was thought such structures could form. Clearly, the Universe had different thoughts about it. 

However, the situation just got weirder: astronomers have found huge and hugely massive spiral galaxy in a JWST image that we see when the Universe was only two billion years old! Everything about it is pretty surprising, including the fact that it exists at all.

The early Universe was more chaotic than it is today. A billion or two years after the cosmos popped into existence, gas and dark matter started to coalesce to form nascent galaxies. Most were small and scraggly, without a lot of obvious structure. These were seen by the bucketload in the first Hubble Space Telescope Deep Field images, which at the time showed some of the most distant discreet objects in the early Universe (remember, light is fast but travels at a finite speed; so it takes billions of years for the light from these objects to reach us, therefore we see them when they were very young). These semi-galactic clumps were closer together back then, which mean collisions were more common, and growth more rapid than we see in galaxies near to us.

But how rapid? Astronomers may have found out by accident. A team of scientists used JWST to look at very distant quasars — galaxies with supermassive black holes in their cores that are gobbling down matter at a furious rate; that matter forms a disk around the black hole that gets infernally hot and glows so brilliantly it outshines all the stars in the galaxy around it — and in one field happened to spot a disk galaxy of unusual size. They named it Big Wheel, for obvious reasons [link to journal paper].

Curious, they scheduled follow-up observations with JWST to pin down its characteristics, and got a shock. The first is that it’s very far away: its redshift is 3.245, meaning we see it as it was when the Universe itself was only two billion years old (or, you can think of it as the galaxy being about 11.8 billion light-years from Earth). But at that distance, its apparent size in the images means it’s a monster, about 100,000 light-years across, comparable to the Milky Way.

That would make it a big galaxy if it were here, near us. But to see one that size that far away, when the Universe was so young, is a huge surprise.

It’s massive, too. Stars in a galaxy rotate around the center at a velocity that depends on the mass of the galaxy. That velocity can be measured by looking at the Doppler shift in the light from those stars in a spectrum, and converted to a mass (the shift depends on the velocity of the stars, which depends on the gravity of the galaxy, which depends on its mass). They find it has a stellar mass — that is, a mass in just stars — of 370 billion times that of the Sun, again, comparable to our own galaxy. (I’ll note the uncertainty in that measurement is large, about ±250 billion solar masses, but it’s still a lot).

Its size is a bit baffling. It’s far larger than other disk galaxies seen at that distance, a real outlier. How did it get so big? And how did it get so big and still retain an obvious spiral pattern?

That’s the weird part. The original discovery image shows Big Wheel is in a dense environment, filled with lots of other galaxies, far more than usual: there are about ten times as many galaxies in the volume of space around it than the cosmic average at that time. So it’s likely part of a protocluster, a cluster of galaxies in the process of forming. The thing is, spiral galaxies were once thought to be difficult to form in such an environment, since collisions between big fragments would disrupt a disk.

Apparently that’s not always the case. Obviously, conditions back then support the formation of a big disk. Some theoretical work has shown it’s possible that collisions can be non-destructive, but things have to be just so for that to work. It’s also possible there’s a lot of gas floating around out there, and Big Wheel is accreting it coherently — in other words, the gas is flowing into it from one preferred direction, helping it maintain its overall rotation. It’s certainly gas-rich; it appears to be forming stars at a rate of a couple of hundred per year, which is a hundred or so times higher than the Milky Way does today!

It’s likely not done growing, either. The gray blob in the disk to the upper left of the center is likely a companion galaxy (in the spectra it shows the same redshift as the Big Wheel but has different characteristics, making it likely it’s a separate object). It’s probably pretty close to the bigger galaxy, and may eventually collide and merge with it.

We don’t really have answers as to how exactly this galaxy could form and grow to such size and maintain its structure this early in the cosmic timeline. That doesn’t mean all our ideas about the Big Bang are wrong! Just that we still need to fill in the details on how this works. Big Wheel is huge, but rare; we now have quite a few observations from JWST of objects at that distance and it stands out. So it may be a singular or exceptional case. But even then, it exists, so we need to figure out how and why. That, in science, is always the fun bit.

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