At the start of cosmic history, galaxies were big clouds of gas, and they grew by turning that gas into stars. If a galaxy runs out of gas, it will stop forming stars and die.
Present-day galaxies have had more than 10 billion years to grow old and die. But this is not true in the early universe: we expect to see very few dead galaxies in the first billion years of cosmic time.
In 2022, the James Webb Space Telescope gave us our first clear glimpse of galaxies in the early universe. What we saw completely defied our expectations: there were too many big, dead galaxies, far earlier than expected.
Astronomers came up with many possible explanations. Some suggested that dark energy – the mysterious phenomenon believed to be driving the universe’s expansion – may have been stronger in the early universe than current theories predict. This would allow galaxies to grow (and die) faster. However, the real solution may be much simpler.
Our new study, published today in the Monthly Notices of the Royal Astronomical Society, reveals an early massive galaxy in the throes of death: its gas is being rapidly blasted into space by a powerful “galaxy wind”, and it may very soon run out of gas. This galaxy offers a new solution to the mystery of what killed big galaxies in the early universe.
Prime suspects for massive galaxy death
There are two ways to eject gas from galaxies: exploding stars (called supernovae) that push gas away, and supermassive black holes that accelerate gas to such high speeds that it escapes the gravitational pull of the galaxy. Both produce fast-moving gas streams that astronomers call galaxy winds.
These winds have long been considered one of the main causes of galaxy death.
Black holes produce faster winds than exploding stars, making them the favoured means for ejecting gas from the largest, most massive galaxies. Many theories suggest that only the powerful winds driven by supermassive black holes can kill the largest galaxies.
However, testing these predictions is hard. As the gas in the wind leaves a galaxy, it becomes very faint very quickly, making galaxy winds difficult to see even in nearby galaxies.
In distant galaxies, they were almost invisible until recently.
Transforming our view of the early universe
Designed to look deeper in space than any telescope before it, the James Webb Space Telescope has transformed our view of the early universe. It allows us to see things that were previously undetectable – including hot, fast-moving gas ejected from early massive galaxies.
For our new study, we paired observations from the James Webb Space Telescope with data from the Atacama Large Millimeter Array, the world’s most powerful radio telescope, which measures cold star-forming gas swept out of galaxies by winds.
Together, these telescopes give us the most complete picture yet of galaxy winds in the early universe.
One galaxy, called CRISTAL-02, stood out to us immediately. We noticed it was forming stars twice as fast as other similar-sized galaxies. Our extremely sensitive observations revealed a huge plume of cold gas extending far away from CRISTAL-02. This plume was almost as long as the galaxy itself – a telltale sign the gas was being driven out of the galaxy.
The wind from CRISTAL-02 was ejecting twice as much gas as the galaxy converts into stars, and this gas was likely travelling fast enough to escape the galaxy. If the wind kept ejecting gas at the same rate, the galaxy would run out of fuel in less than 100 million years – a blink of an eye in cosmic terms – forming a massive dead galaxy less than 1.5 billion years after the Big Bang.
Paradoxically, the wind appeared to be driven by the same intense star formation that was making the galaxy grow so quickly.
Rebecca Davies
Cosmic collisions may hold the answer
To complete the picture, we need to understand why CRISTAL-02 was growing so fast in the first place.
The answer may lie in the fact that CRISTAL-02 is not a single galaxy, but multiple galaxies in the final stages of a cosmic collision. During such collisions, gas funnels towards the galaxy centres, triggering strong bursts of star formation.
In the present-day universe, galaxy collisions are relatively rare: they are seen in only a few percent of galaxies. But one billion years after the Big Bang, the universe was far more compact, meaning galaxies were packed much closer together.
Recent studies suggest around 40% of big galaxies in the early universe are in the process of merging. Some of these galaxies will likely face a similar fate to CRISTAL-02: undergoing frenzied bursts of star-formation, followed by powerful winds that lead to their deaths.
Our findings show that powerful winds capable of killing galaxies do not originate exclusively from supermassive black holes: they can also be triggered by the intense star-formation that causes galaxies to grow rapidly.
If many early galaxies collide and experience rapid growth, then it may not be surprising at all that we see so many dead galaxies in the early universe. CRISTAL-02 offers a natural solution to the mystery of why these massive galaxies live fast and die young.
