The ALMA image of the very distant galaxy SPT0418, distorted into a ring due to the gravity of an intervening galaxy. Credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.
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The ALMA image of the very distant galaxy SPT0418, distorted into a ring due to the gravity of an intervening galaxy. Credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.

Why is there a normal galaxy sitting at the edge of the Universe?

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Sep 7, 2020, 4:33 PM EDT (Updated)

Aided by a trick of gravity, astronomers have found a normal galaxy. Big deal, right? The thing is, where they found it is not normal: The light we see from it left the galaxy 12.4 billion years ago, meaning we’re seeing it as it was when the Universe itself was only 1.4 billion years old!

That’s what makes this so weird. A normal galaxy has no business being there when the cosmos was so young. But yet, there it sits.

The galaxy is called SPT–S J041839–4751.9 (let’s call it SPT0418 for short) and was discovered in a sky survey using the South Pole Telescope, which looks at light in the millimeter wavelengths, far outside what our eyes can see. The galaxy was flagged as interesting at the time, since it appeared to be dusty and cranking out new stars.

Follow-up observations were made using ALMA, the Atacama Large Millimeter/submillimeter Array, which is far more powerful and has much keener eyesight, so to speak. What they saw is one of the first weird things about this normal galaxy.

The ALMA image of the very distant galaxy SPT0418, distorted into a ring due to the gravity of an intervening galaxy. Credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.

Normal, eh? That looks pretty peculiar to me: It’s a ring! Except that’s a mirage of sorts. By coincidence, almost exactly in a line between us and SPT0418 is another galaxy, located about 3 billion light years from us. As the light from SPT0418 passed this closer galaxy on its way to us, its path got bent by the gravity of the intervening galaxy. We call such a beast a gravitational lens, and a lot are known. They distort the shape of the more distant galaxy, sometimes into multiple blobs, or arcs.

Distorting the light into a ring needs near perfect alignment, and that’s pretty rare. Even though the shape is warped, a big advantage is that the light gets amplified, making faint galaxies visible — there’s no way this extremely distant galaxy would be observable in any detail without the lens.

Gravitational lensing was predicted by the equations of relativity (they’re sometimes called Einstein rings), and in a sense the equations can be reversed to figure out what the lensed galaxy really looks like. The astronomers did this modeling, and what they found is actually pretty cool.

The shape of the galaxy SPT0418 constructed using model s of how gravitational lensing behaves. It appears to be a relatively normal disk galaxy, despite its youth and extreme star formation rate. Credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.

The galaxy looks … normal. Like a disk-shaped galaxy you’d see nearby in the Universe, even like the Milky Way (though only about a quarter the mass of our galaxy). It has a flat disk of stars, gas, and dust, and even a central bulge of stars like our galaxy does, too.

Unlike ours, though, this galaxy is particularly fecund. It’s making stars at a rate of 350 times the mass of the Sun per year! That’s a lot, as much as two hundred times the rate the Milky Way makes stars. SPT0418 is what we aptly call a starburst galaxy.

But that’s why it’s so weird. Theoretical studies of how galaxies form and evolve over time indicate that such starburst galaxies should be pretty chaotic-looking, chewed up by the turbulence created when stars are cranked out at that rate. Massive stars blow powerful winds, supernovae detonate like flashbulbs, and all that energy released should stir the gas up in the galaxy, making it irregularly shaped (plus, that early in the age of the Universe there just hadn't been a lot of time to get organized). Most distant galaxies we see are indeed blobby. The fact that this galaxy has a disk that appears to be pretty calm and stable is bizarre.

Artwork depicting the Wolfe Galaxy, a massive disk galaxy similar to the Milky Way that was already well formed when the Universe was 1.5 billion years old. Credit: NRAO/AUI/NSF, S. Dagnello

Interestingly, it’s not alone. Earlier in 2020 astronomers announced they found a disk galaxy at about the same distance, called the Wolfe Galaxy. Like this one, it’s not understood how it can exist. Clearly, the theoretical models are wrong, or at least (and more likely) incomplete. Obviously, there’s more to learn about galaxies that exist at the edge of the observable Universe.

As I mentioned, observing galaxies like this so far away is nearly impossible without gravitational lensing, which is random and sparsely spaced across the sky. That means it’s hard to look this far away and see what’s going on, so we have to rely on our theory. But then we build ever better telescopes like ALMA, and find out that things are different than we expected. That happens pretty often. So finding a weirdly normal galaxy in a normally weird place is, oddly, weirdly normal!

The job now is to find out why.

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