JThursday, November 17, NASA lifted a corner of the veil on the first galaxies in the universe by announcing the discovery, by the James-Webb Space Telescope (JWST), of two specimens observed approximately 450 and 350 million years only after the Big Bang, in the outer regions of the supercluster of galaxies Abell 2744, visible in the constellation of the Sculptor. The one seen 350 million years after the Big Bang is to date the youngest ever detected, the previous record being 400 million years. The French astrophysicist Nicolas Laporte, researcher at the University of Cambridge in the United Kingdom, specialist in the question, answered the questions of the Point.
Point : Fifty million years, is that a great leap forward for astronomy?
Nicholas Laporte: Clearly, no. We stay more or less in the same type of galaxy. What is really interesting in this discovery is the resolution with which it is possible to look at these first galaxies! Hubble only saw them as spots. There, with the James-Webb telescope, because it has a very, very high resolution, we are able to see almost inside. I’m not going to say that we observe stars in the process of forming, but we are able to see that these very first galaxies are many small regions with lots of stars that are in the process of merging. And that is the great discovery!
You should also know that we are talking here about scientific results that were submitted only a few days after the publication of the very first data from the telescope. Even if they have only just been accepted. In other words, it was really to tell us: this is what we can do with James-Webb. So, even if it’s not a huge leap forward, we’re keeping great promises.
It seems to me to understand that these galaxies are surprisingly bright. What does that mean ?
It’s true ! If you remember, we discussed the very first image of James-Webb and I told you that we saw a lot of red galaxies. However, when a galaxy is red on the image of a telescope, it means that it is far away, or that it has a lot of dust. In fact, what we realize, by analyzing the James-Webb data, is that there are many more distant galaxies than we expected to see, including brighter galaxies. And the two galaxies we’re talking about are actually a bit brighter than expected. That means they have a lot more stars than we thought. That is what is interesting.
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Does the fact that they have more stars suggest that they were born earlier than we thought?
I am convinced of it. Now, with the data they had, they can’t say. They don’t have enough information to measure the age of the galaxy. So there are two hypotheses for having such a bright galaxy. Either we say that it is a burst of star formation and that all the stars formed at the same time. That’s a hypothesis in which the galaxy would be young. The second hypothesis is to say that the galaxy has formed stars at an almost constant rate for several million years. And that would mean that we have an old population of stars and that these galaxies are a little older. They’re going to need a bit more data, with Webb, to conclude and say which of the two scenarios is the correct one.
These two galaxies were spotted in a very restricted area of the sky. Is it any wonder they are so close together? Does this mean that their concentration is higher than expected, or not?
There is something called cosmic variance: that is, if you look at one place in the universe, it may not necessarily be exactly the same as another place in the universe. universe. Here we are talking about the first two James-Webb fields of view. But Webb’s field of view is really very small! Two minutes of arc by two minutes of arc is like holding a pin at arm’s length reaching into the sky, and roughly what JWST will see is the top of the head of pin. Drawing conclusions on only two small fields of view is a bit complicated. In my opinion, we will have to wait several months for Webb to make observations over a much larger area before we can have strong constraints on the density of these galaxies.
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Finally, are these galaxies very different from those we know?
Well yes ! Because they didn’t have time to do arms, for example. The beautiful spiral arms that we observe in the Andromeda galaxy or even in the Milky Way should not have existed at the beginning of the universe. There, with Webb, we are seeing it. What we observe are very small galaxies which are gradually coming together. Then they will merge to give slightly larger galaxies. But, for now, there is no arm, there is no bar in the middle, etc. They are formless galaxies. It takes perhaps a billion years for them to begin to harbor somewhat robust structures.
When you started this interview, you were talking to me about promises. Is the JWST currently holding its own?
Oh yes! The pictures are fabulous. The resolution with which we can see these galaxies forming is truly incredible. And we are only talking about images here, but in spectroscopy Webb can show us elements, metals that we have never seen before in galaxies. We can see and precisely measure their distance. That too is quite incredible. In the meantime, with the launch of Artemis taking place two days ago, James-Webb can no longer observe. All of NASA’s bandwidth is now reserved for this mission. We had a program that was supposed to start today and, bad luck, we were told: you’re going to wait a fortnight! But hey, we gladly leave our place to the Moon… (Laughs)