'Dead' galaxy stopped forming stars 13 billion years ago

With the help of the James Webb Space telescope, astronomers have spotted a galaxy that suddenly stopped forming new stars more than 13 billion years ago. The ‘dead’ galaxy was just 700 million years old - which makes it the oldest such galaxy ever observed. To find out more, I went to meet Dr Francesco D’Eugenio from the Kavli Institute for Cosmology at the University of Cambridge…

Francesco - So the universe was born from the Big Bang, and then it started as a uniform distribution of gas. But if we look out today, the universe is made of galaxies. It's mostly empty and is dotted with galaxies. So understanding how it went from this uniform fireball of gas to what it is today is one of the main tasks that we're trying to understand in contemporary astrophysics.

Chris - How can you see back in time in that way then? Because what you are saying is, 'I want to get a glimpse back to not long after the universe formed to see how that very homogeneous smooth thing became aggregates of material that is going to be segregated into galaxies.' But that was millions, billions of years ago.

Francesco - Exactly. So the key here is that the speed of light is very, very high, but it's finite and it's this that permits us to look back in time. The light that we see from the sun now left the surface of the sun about five minutes ago. So when we look at the sun, it's as if it was five minutes ago and, as for the closest star, we see it now as it was four years ago. So the earliest galaxies enable us to see how the universe looked sometime after the Big Bang. The most distant galaxy, we can see it as it was 300 million years after the Big Bang. Of course, the galaxy now is completely different.

Chris - How can you interrogate the information coming to us from 13 billion years ago to understand what the structure and the dynamics of that very early galaxy would've been then.

Francesco - The key information here is that those galaxies are made of the same matter and obey the same laws of physics that are observed in the local universe. So for example, we can look at signatures of chemical elements and the colour of the light emitted by these chemical elements which tell us about how distant the galaxy is and also tells us the intensity of these various colours. If it's bluer or redder, let's say, for particular elements, that tell us about the chemical abundances and also the physical processes actually that excite this light.

Chris - I get it. So you're almost doing a molecular postmortem on what that galaxy looked like, what the stars in it were making, what they were made of, the abundance of different chemicals. You can tell all that even though it was 13 billion years ago and the universe wasn't very old.

Francesco - Exactly.

Chris - You must presumably then have started with an idea or a theory about what you thought the universe and its galaxies looked like at that time point. And then, now you've actually got one that old and got this data, you can ask, is it different from our theory and if so, how different and why might that be? Is that the direction you took?

Francesco - So we expected, for example, that most if not all galaxies will actively start forming in what we call a starburst; so very, very high rate of formation of stars. We're talking about tens or hundreds of new suns per year in those galaxies. But instead we found that some of these galaxies have stopped forming stars, and this is remarkable because in this epoch, the universe is replete with gas. There is a gas abundance and this is the fuel for star formation, so galaxies are the ideal place to compress this gas and start forming stars. So in some way it's akin to seeing a smoker in a sea of petrol and nothing is happening. We don't know if something removes the fuel so that's why there's nothing burning or if for some reason the fuel refuses to catch fire.

Chris - Do you know if it's the rule or the exception?

Francesco - So far we think it's the exception. We have only found one target, one galaxy that has this particular property. This one is probably the most beautiful example but we recently got awarded more time to look for similar targets to confirm and understand exactly what their incidents are. In astrophysics, what we try to do is to understand time dependent processes, but only having a snapshot frozen in time if you want, it's like trying to understand human life from a picture of the whole world. So the number of this galaxy will tell us, for example, how often this occurs and therefore what are the likely physical processes that can power it, how fast this occurs. And so this can tell us, for example, if it's due to energy released by matter, falling onto super massive black holes.

Chris - Doesn't it blow your mind a bit though, even though you are an expert in the subject, to think that you are seeing something from so soon after the whole universe began to exist? It really makes me feel a bit of shivers down my spine when you start to have conversations like we're having.

Francesco - It is indeed very, very, I think, humbling to think how big the universe is and how small we are. And I think that puts it in perspective a bit our life and that our common history started so long ago, the elements we are made of, we as in our bodies are made of, for example, blood in our veins contains iron that was made inside stars and then circulated in the galaxy, ended up on this planet and finally in our bodies. So our story in some sense our common history started so long ago.