The space-based observatory JWST has been revolutionizing astronomy in a lot of ways, including how we see the very early Universe. Because the Universe is expanding, very distant objects undergo a serious redshift, where their light is stretched to longer wavelengths. At the same time, we see these distant objects as they were when they were very young, because it’s taken that light many billions of years to reach us.

Young stars and active black holes — both expected to exist and be very bright in young galaxies —  blast out a lot of ultraviolet radiation. By the time that light gets here it’s been redshifted to infrared. JWST is a huge telescope with very sensitive instruments, and it’s been finding many of these very distant high-redshift galaxies. 

I urge you to read an earlier article I wrote in issue #730 that explains these concepts. 

But now we have something new in this field.

Astronomers used JWST to stare at one spot in the sky for a long time, to detect these distant galaxies. The project is called JADES, for JWST Advanced Deep Extragalactic Survey. One of the galaxies initially identified as being very distant is called JADES-GS-z13, where z13 means it’s at a redshift of 13, one of the highest ever seen. That means any feature in it has its wavelength shifted by a factor of 14 (I know that’s weird, but it’s part of the definition as I explain in that link above). This huge redshift means we’re seeing this galaxy as it was when the Universe was only 330 million years old. It’s 13.8 billion years old now, so this was a long time ago, when the cosmos was very young.

Following up on the discovery of GS-z13, astronomers used JWST to take spectra, where they can examine the colors of the object in greater detail. What they found is amazing: a Lyman-alpha line just booming out from the background [link to journal paper].

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OK, so what’s that? In a hydrogen atom, electrons can occupy certain energy levels. In old school descriptions these are depicted like planet orbits, but that’s not a good model. They’re more analogous to a staircase: electrons can exist on any step up from the bottom, but not in between (you can take one step up, or two or three, but not one-and-a-half steps). It takes energy to raise an electron from one level to the next, and the atom releases energy if an electron drops from a higher to a lower level. Because the steps are fixed in energy, it takes an exact amount of energy to move from one level to another.

This energy comes in the form of photons, particles of light. When an electron drops down from a higher level it emits a photon with a certain energy, and if you see that photon coming from, say, hydrogen gas you know which levels the electron occupied. That’s important, because it takes energy to pump electrons up to some high level, which they release when they eventually move back down. This can tell you about the environment of the gas, like if it’s hot or cold, ionized (missing one or more electrons), and more.

The energy of a photon and its wavelength are simply related, so if you measure one you can get the other. Typically, telescopes measure the wavelength.

So, in a hydrogen atom, the photon released when an electron drops from the second energy level down to the first is called Lyman-alpha, and it has a wavelength of about 0.1216 microns, in the ultraviolet. This is very commonly seen in hot environments loaded with hydrogen, like massive stars and black holes gobbling down matter.

That’s what was seen in GS-z13: a Lyman-alpha line, redshifted by a factor of 14 to a wavelength of about 1.71 microns. That indicates the galaxy had a lot going on, either a lot of extremely massive hot stars, or a supermassive black hole in its core where matter falling in is blasting out ultraviolet radiation.

But…we shouldn’t be seeing that light! Why not? Because things were different back then, 330 million years after the Big Bang.

At that time, the Universe was filled with hydrogen. Initially, when the cosmos was a few hundred thousand years old, that hydrogen was so hot it was ionized. When hydrogen loses an electron it becomes transparent to UV and visible light. But as the Universe expanded it cooled, so electrons could combine with hydrogen, making it electrically neutral (for historical reasons, this era is called recombination). It became opaque to light.

Some time after this happened, something like six hundred million years later, massive stars and active black holes spewed out so much UV light that the hydrogen became ionized again. Astronomers call this era reionization, and it’s when the Universe became transparent again. Expansion continued, and even though the Universe cooled, it also became less dense, and it became harder for electrons and protons to recombine again. The cosmos remains transparent to this day.

That’s kinda important! If the Universe were still opaque, we wouldn’t be able to see anything.

And that’s where GS-z13 has handed us a surprise. When the Universe was 330 million years old, it was opaque! That Lyman-alpha light shouldn’t be reaching us; it should’ve been absorbed by all that hydrogen floating around back then. Yet it wasn’t. It somehow burst through that stuff and made it to us. How?

That’s the key question. One part of the answer is that the objects in the galaxy were so luminous that probably carved a transparent bubble of ionized hydrogen around the galaxy that could’ve been pretty big, making it easier for UV photons emitted later to travel farther. Also, if it were really, really luminous then some of those photons would be able to make it far enough out that they eventually get to the point where the Universe became transparent, and it was free sailing from there to here.

How luminous? Making some pretty conservative assumptions, the astronomers calculate that GS-z13 was blasting out 10 billion times the Sun’s luminosity in just UV photons. That’s… a lot. Holy yikes, that’s bright. Mind you, we see this galaxy when it was extremely young. Somehow, in just 330 million years, it was able to organize itself enough to become one of the brightest objects in the early Universe, powerful enough to clear the cosmic fog around it locally, and still have enough left over to pump photons out that could make it here, after 13.5 billion light-years.

That in itself is a big deal scientifically, but — like every discovery in science ever — it raises more questions than it answers. What’s making all these photons, stars or a monster black hole? How many are really making it out (crucial to determining how luminous the galaxy actually is)? How do galaxies get their act together so quickly to be able to do this? How many such galaxies like this exist? JWST only looked at a small portion of the sky, implying that there are a lot of galaxies like GS-z13 in the sky. Tens of thousands, certainly. Also, what does this tell us about the era of reionization? Can we get a better idea of when it happened and how long it took? That will tell us a lot about overall conditions in the Universe when it was less than a billion years old.

Even with JWST, observations like this are tough, taking hundreds of hours of exposure times. Given how oversubscribed the observatory is, it may be a while before we see more extremely deep images and spectra like this taken in another part of the sky. I hope time is carved out to do so though. When we did this with Hubble we discovered all sorts of incredible things. With JWST’s infrared vision able to peer even more deeply into the Universe, what other surprises await?

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