THERE were some exciting goings-on in astrophysics last year. The one that probably got the most attention was the question of whether NASA’s latest flagship observatory, aka JWST or what I like to call the Just Wonderful Space Telescope, had upended what we know about the timeline of galaxy formation. The hints that there might be a problem with our understanding of how galaxies take shape came almost as soon as JWST started returning data. In 2022, the world watched as astronomers learned that galaxies seemed to form earlier, or at least faster, than expected in the early universe.
The story continued in 2023. In September, researchers led by Kasper Heintz they had discovered an early universe breakdown in a well-established correspondence between star formation and galactic gas metals. In astronomy, the word metals means any element more massive than helium. The team had been studying the rate at which stars formed in the early universe and the amount of metals in galaxies. There should be a clear correspondence because it is via nuclear fusion in stars that any element more massive than hydrogen or helium typically forms in the universe. The latest JWST data suggests that, in the early cosmos, the amount of metals is less than expected, based on how many stars already exist.
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How this can possibly be is now a new question for astrophysicists and cosmologists to reckon with. The authors of the 2023 report propose that this indicates that galaxies in the early universe were in effect being watered down by the non-metallic gases that make up the intergalactic medium, or IGM, that surrounds them. It is possible you have never heard of the IGM before because it isn’t visually exciting the way other visible matter formations are. It isn’t bright like stars and it isn’t a weird space-time formation like black holes. But the IGM is thought to be home to about half of the visible matter in the universe, and because it is hard to see, like dark matter – the as-yet-undetected form of matter that makes up some 85 per cent of the total mass of the universe – we have to infer a lot about it by looking at galaxies.
The IGM is quite unlike dark matter though, because it does actually radiate, and when it is backlit by X-rays coming from bright objects like galaxies with especially powerful black hole engines at their centre, we can take a look at it. There is still a lot to be learned, which is part of what makes this JWST mystery so alluring. It is possible that the low metallicity in these early galaxies is telling us something about the history and structure of the IGM and how galaxies have interacted with it over time. Thus, JWST has opened a host of new questions about star formation, galactic gases and extragalactic gases.
This is a great example of how doing science doesn’t lead to the end of science, but rather to more questions in need of answers. And while these are tantalising datasets, they weren’t the most exciting galaxy-related result in 2023, at least not for me.
That honour goes to the apparently showing that a satellite galaxy of the Milky Way, the Small Magellanic Cloud, should be thought of as composed of two distinct stellar systems superimposed on one another. My first response to this new paper on social media was: “Huge if true!” This was a bit of a joke, not only because it is the Small Magellanic Cloud and not the large one, but also because galaxies are both massive and large in spatial extent.
This result is especially exciting for me as someone who has spent the past few years trying to understand the relationship between galaxies and their dark matter halos. Studying the visible components of Milky Way satellite galaxies is one of the best ways to gain insight into this galaxy-halo relationship, since we think nearly every galaxy, even the satellites, lives inside its own dark matter halo. By looking at how stars are moving inside the galaxy, we can reverse engineer the impact that dark matter is having and in the process maybe even learn something about the fundamental nature of this matter.
As of writing, this result is so hot off the press that it has been accepted into The Astrophysical Journal, but has yet to be formally published there. My expectation is that there will be robust follow-up this year. There are a lot of parts to this research and scientists will want to check and recheck whether the assumptions and calculations make sense.
My prediction? In the end, this is an exciting result that will not only hold up, but also transform how we think about our galactic neighbourhood. As a member of the new Vera C. Rubin Observatory (which is set for first light in 2024 and 2025), I am keen to see if this has shifted the science we thought we were going to do. What a time to be alive and doing cosmology!
Chanda’s week
What I’m reading
I am listening to actor Peter Ganim read Nathan Thrall’s A Day in the Life of Abed Salama: Anatomy of a Jerusalem tragedy.
What I’m watching
I absolutely loved the horror-comedy film Vicious Fun.
What I’m working on
I’ve been trying to teach myself some quantum optics!
Chanda Prescod-Weinstein is an associate professor of physics and astronomy, and a core faculty member in women’s studies at the University of New Hampshire. Her most recent book is The Disordered Cosmos: A journey into dark matter, spacetime, &Ěýdreams deferred
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