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For many years, I worked on a camera on board Hubble that took spectra. This means it took incoming light from some celestial object and separated it into different wavelength bins, showing how bright it was at these different wavelengths — you can think of these as colors, but instead of the seven colors of the rainbow we can actually distinguish thousands, sometimes hundreds of thousands, of very thinly sliced colors with such cameras. So not just red, but hundreds of hues of red. This is important because different elements emit light at different wavelengths, so spectra can tell us what a cosmic object is made of.

I did a lot of work with these spectra, and saw them from objects all across the astronomical dictionary and from objects as close as the moon to galaxies billions of light-years away.

So when I say I saw this spectrum taken using JWST’s MIRI (Mid-InfraRed Instrument) and my jaw literally dropped open, you can understand that this is an extraordinary observation [link to journal paper].

OK, let’s take a sec for the background info. 

JADES-GS-z14-0 is a galaxy that is so far away that the light we see from it has been traveling across the universe for a staggering 13.5 billion years — we see this galaxy as it was when the universe itself was a mere 300 million years old. Before the JWST infrared space telescope launched we though that galaxies at this time were pretty unorganized and “primitive”, with stars just getting started. However, it didn’t take long for JWST images to show that some of these galaxies were already well evolved; for example some sporting supermassive black holes in their cores that were far larger than we thought possible given the limited time they had to grow (this is still an issue astronomers are trying to explain). 

Now, right off the bat JADES-GS-z14-0 is an incredible object. We measure the distances to these galaxies using their redshifts: the expansion of the universe sweeps these galaxies away from us, which imparts a tremendous Doppler shift to their spectra. That means that the wavelength of light we see from them has been redshifted, the wavelength lengthened. When I worked on Hubble we were struggling to see objects with a redshift of z=6, which means the wavelength was expanded by a factor of 7 (I explain all this in detail in BAN #730).

Almost immediately after launch, JWST was seeing galaxies with far higher redshifts. JADES-GS-z14-0 has a redshift of z=14, which still gives me the heebie jeebies to write. That’s a helluva large redshift, meaning the light we see is really stretched out. What that means in practice is that some of the usual spectral features we see in visible light, like light emitted from oxygen and hydrogen that appears green and red to our eyes is shifted into the far-infrared.

…which is just where JWST sees. MIRI has an observing mode that can take spectra out to wavelengths of 10 microns — over 13 times longer than the human eye can detect— which is just what’s needed to see these features in JADES-GS-z14-0.

So. In the image above, the top part shows the actual spectrum of the galaxy. This is not an image like you think of one! The vertical axis is indeed a spatial direction (like the y-axis, or rows, of an image), but the x-axis is wavelength. The blobs of light you can see are where the galaxy is bright at those wavelengths — the bottom part shows that in graph form, where the y-axis is brightness. So, for example, there is a bright blob at 7.5 microns, and another fainter one at 10 microns. When you account for the huge redshift, those blobs are actually from oxygen (emitting at about 0.5 microns (green) in the galaxy’s frame of reference) and hydrogen (about 0.65 microns (red)). The data are a bit noisy, but this light has been traveling for 13.5 billion years, so it’s faint. This spectrum represents an amazing 34 hours of total observations, the longest exposure spectrum ever taken with MIRI. The fact that we can see the oxygen and hydrogen emission at all is incredible.

These data are very useful! The physics of how hydrogen and oxygen gas in galaxies emits light is very well understood, and can be used to determine some physical characteristics about the galaxy. The most amazing thing, in my opinion, is that the astronomers who took these observation show that the amount of oxygen in JADES-GS-z14-0 is about half the amount we see in the sun.

Why is that amazing? When the universe was very young, all that existed essentially was hydrogen and helium. Heavier elements like oxygen have to be made inside massive stars, which then explode as supernovae and scatter those elements into space. That takes time, and over the eons more and more heavy elements have built up. We use the sun as a standard, and compare the heavy element contents of other stars and galaxies compared to it. 

For JADES-GS-z14-0 to have that much oxygen in it just 300 million years after the Big Bang is incredible. That’s faster than expected, which means stars are somehow more efficient at making and distributing heavy elements than we thought. That may be because we don’t know much about the first generation of stars in the universe; they were very different than stars today, and while there is some theoretical understanding of them, they’ve not yet been observed — they are far away and extraordinarily faint — so it’s hard to check our models for accuracy. Once we actually start to see some I’m sure we’ll get better at figuring them out.

And what this spectrum is showing us is that we still have a lot of work to do in this field. Heck, the spectrum is also showing us there’s work to be done just analyzing this spectrum. See all that light to the left of the oxygen blob? That’s probably the aggregate light from stars in that galaxy, and there’s plenty to analyze there, too (the authors say they have another paper coming out soon working on that).

The extreme early universe is unknown territory for this type of observation, and it’s clear there’s a lot of work to do to understand it. But this is what astronomers eat for breakfast. There will be more observations like this — hey, JADES-GS-z14-0 is only the second most distant galaxy known, and that record likely won’t stand for all that long either — and as time goes on we’ll start to figure all this out. Bet on it.

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