JWST bags the most distant galaxy ever seen… and hoo boy, it’s a mystery

How did a massive galaxy form so soon after the Big Bang?

June 4, 2024 Issue #730

Astronomy News

It’s a big Universe. Here’s a thing about it.

Drilling down on an already incredibly deep survey of the Universe made using JWST, astronomers have spotted a record-breaking galaxy: It’s the most distant one ever seen, and the fact that it exists at all is a mystery. It was already massive and extremely bright just 300 million years after the Big Bang! That’s incredible, and will push our understanding of star and galaxy formation past what we used to think was possible (or at least likely).

CAVEAT: The three journal papers written about this object and its implications are online but have not yet been peer reviewed [Paper 1, Paper 2, Paper 3]. That doesn’t mean they’re wrong, any more than it means they’re right; it just means that outside, independent review hasn’t yet been done. My guess is the papers will be reviewed and accepted pretty soon, but even then it’s possible there are other explanations for the surprising results. I suspect they’ll be borne out by the evidence, but this is science and it’s hard to be 100% certain, so take all this with a tiny grain of NaCl.

 

Background

The Universe is expanding. This has a lot of implications, including three big ones: The cosmos had a beginning, we see distant galaxies as they were long ago, and the light from them is redshifted.

The first of those has been pretty well established, and we know the Universe is roughly 13.8 billion years old and has been expanding ever since. If you want the particulars, why, I know a guy.

The second is due to the finite speed of light. It takes time for the light from an object to travel across space and be seen by us, so the farther away something is, the earlier in its history we see it. Light from the Sun takes a little over eight minutes to reach Earth, so in a sense we see it as it was eight minutes ago. When we see a galaxy 13 billion light-years away, we see it as it was when the galaxy — and the Universe itself — was very young. Galaxies formed as huge clouds of gas condensed and collapsed, creating stars by the millions. We’re not sure how long this process took (more on that in a sec) but probably a few hundred million years. Over time galaxies grew bigger, merging with other galaxies to grow even more. Disk galaxies like the Milky Way therefore looked very different when they were young, so we expect distant galaxies to look blobby and lumpy. That’s been supported by countless observations.

The third thing is fun. You’re familiar with the Doppler shift, like when a motorcycle drives by and the engine noise goes “EEEEEEEEEEE-ooooooooooo”, dropping in pitch when it passes. Light does the same thing: the light from an object approaching you will have its wavelengths compressed (what astronomers misleadingly call blueshifting) and one receding will have its wavelengths elongated (redshifting). The faster it’s moving away the longer the wavelength gets.

Astronomers denote this with the letter z. An object that’s wavelength is redshifted by a factor of two is said to be at a z of 1 (local space is z=0). A factor of three is z=2, and so on. It’s not a linear scale; in other words a z=4 object is not twice as far as a z=2 object. The steps are big at first (the difference between a z=1 and 2 is large) and gets smaller with each step (so a z=9 galaxy is only a little bit farther than one at z=8).

Redshift gives us a relative distance. The actual distance depends on lots of things, like what model you use for how the Universe expands. But the higher the number, the farther the object, the longer it took its light to reach us, and the younger we see it (if you want to take this to its logical endpoint, a z=infinity would be something right at the moment of the Big Bang when it was 0 years old and it took 13.8 billion years for its light to reach us).

The farthest galaxies we’ve seen with Hubble are at a z of roughly 10. Most young galaxies, though, emit light so redshifted that Hubble can’t see them, which is where JWST comes in. It sees exactly that kind of light, so astronomers were hopeful and excited to see what it might uncover. Right after JWST launched it took a deepish image of the sky, and some preliminary analysis indicated high-redshift galaxies, but most weren’t confirmed, or were later shown to be at lower z

A deeper survey called was taken in late 2023 and early ’24. JWST was pointed at a patch of sky in the constellation Fornax, looking almost directly out of our Milky Way galaxy; that minimizes the amount of local junk (stars, dust, and so on) that’ll interfere with seeing extremely distant galaxies. The image is incredible; tens of thousands of galaxies can be seen. But one caught astronomers’ attention: JADES-GS-z14-0.

 

The Distant Baby Galaxy 

Thousands of galaxies dot an image, with one highlighted by a square and zoomed in. It looks like a small red smudge.

Part of the JADES survey with the distant galaxy highlighted. Credit: NASA, ESA, CSA, STScI, B. Robertson (UC Santa Cruz), B. Johnson (CfA), S. Tacchella (Cambridge), P. Cargile (CfA).

 

[Click on that image (or here) to get access to far higher resolution images, including 12,000 pixels on a side!]

JADES (JWST Advanced Deep Extragalactic Survey) and GS (Great Observatories Origins Deep Survey-South) are surveys of the sky, and z14 — spoiler alert! — is an object that looked to be at a staggering z=14. This was found by looking at its colors; how much light is seen in different filters, which can be used to get a good estimate of the redshift. But it was very close to another galaxy that is much closer to us, so interference was a concern.

So the astronomers followed up with more observations, this time targeting the galaxy specifically and reducing interference as much as they could. They took a spectrum, which breaks the light up into individual wavelengths, like a rainbow. Galaxy spectra have a hard break at a wavelength of about .1216 microns (in the ultraviolet), because light with shorter wavelengths than this get absorbed by hydrogen. You see nothing with short wavelengths, then suddenly BLOOP: a sharp rise in brightness at longer wavelengths. This is a very characteristic feature called the Lyman Break.

At very high redshifts this break is shifted to infrared, where JWST can see it. And the spectrum of the galaxy shows this feature redshifted by a factor of 15.3 in wavelength, meaning z=14.3. 

A graph showing the spectrum of the galaxy, its brightness versus wavelength, with it being essentially 0 until about 1.8 microns, where it rises suddenly.

The spectrum of the galaxy shows the Lyman Break, where no light is seen at shorter wavelengths (to the left) and then a sudden rise in brightness. The redshift of the break indicates it’s shifted by a factor of 15.3. Credit: NASA, ESA, CSA, J. Olmsted (STScI). Science: S. Carniani (Scuola Normale Superiore), JADES Collaboration.

 

The Problem and the Opportunity

Yikes. Assuming this is all correct, we see this galaxy as it was about 300 million years after the Big Bang! That’s far, and young. Stars wouldn’t have had much time to form, yet there they are. The galaxy is about 1,600 light-years wide (much much smaller than the Milky Way, as you’d expect for a baby) but extremely bright, indicating it already has hundreds of millions of stars in it. The astronomers rule out the possibility that we’re seeing a big black hole gobbling down matter and blasting out light, because that happens over a much smaller volume of space, dozens of light-years, not thousands.

A closer look at the JADES images shows galaxies in more detail, with JADES-GS-z14-0 highlighted.

Zoom in on the region near JADES-GS-z14-0. Credit: NASA, ESA, CSA, STScI, B. Robertson (UC Santa Cruz), B. Johnson (CfA), S. Tacchella (Cambridge), P. Cargile (CfA).

 

Still, yegads. Current models of how galaxies form don’t show them being this organized that early, so this is a problem. Mind you, it’s not a deal breaker for science; it just means the models need to be adjusted. The models are constructed using equations we know about things like how gas flows, how stars interact with that gas, and so on. Astronomers then plug in numbers from observations like what conditions were like in the early Universe (gas density, temperature, and the like), and up until now we didn’t know galaxies could form this quickly.

So it’s likely there are some conditions in the young cosmos we don’t understand quite yet. Theorists will work on the models, using different numbers and conditions, and see which ones fit the observations best. They can then make predictions about what else we can see, and those can be tested by more observations. These JWST findings are brand spanking new, so it may be a while before they’re fully grokked.

We do know more about this galaxy, too.

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