Astronomers have found hidden signs of past upheaval inside distant galaxies that shut down their star formation long ago. Seen as they were around nine billion years ago, these recently quenched galaxies – systems that had recently stopped making new stars – look calm at first glance.
However, the James Webb Space Telescope has looked beyond the smooth surface and has spotted faint scars of a violent past.
The finding gives astronomers a fresh handle on a long-standing puzzle, namely why the most massive galaxies in the young Universe stopped growing so abruptly.
It supports the view that a single, violent event, most likely a collision between galaxies, can switch off star formation across a whole galaxy in a short span of time.
Why galaxies go quiet
Galaxies grow by turning gas into stars, and for billions of years the largest ones did this at a furious pace. Then some of them stopped.
Working out what flips that switch, and why it happens so fast in the biggest galaxies, has puzzled astronomers for decades.
The galaxies in this study belong to a rare class known as post-starburst galaxies – systems caught soon after a sudden burst of star formation was cut off.
An international team led by Dr. David Maltby, an astronomer at the University of Nottingham, used deep Webb images to examine about 120 quenched galaxies, comparing them with roughly 3,000 other galaxies.
Peak star-building activity
In Webb’s images, most of these quenched galaxies look quiet and orderly, their light fading smoothly outward from a bright core. They also tend to be small, cramming the mass of a mature galaxy into a region only a few thousand light-years wide.
These galaxies come from a stretch of cosmic history often called cosmic noon, roughly nine to 11 billion years ago, when the Universe built stars faster than at any time before or since.
Pinning down how the giants of that era died has been a stubborn problem, and astronomers have long wrestled with why such galaxies stopped forming stars.
“This was the epoch of peak activity in the Universe, when many of the most massive galaxies we see today were formed,” said Omar Almaini, a professor at Nottingham who led the survey that flagged the galaxies.
Signs of disturbance
By most measures, the recently quenched galaxies really are as settled as they look.
The team gauged how lumpy and lopsided each one appeared, and on those counts the quenched galaxies matched fully retired galaxies that had stopped forming stars long before.
However, when the researchers subtracted a smooth model of each galaxy and studied the faint light left over, the most massive quenched galaxies in the early Universe came out distinctly more lopsided than their settled counterparts.
No one had measured this before across so many recently quenched galaxies. The lopsidedness sits hidden beneath an otherwise smooth glow, a level of structural disturbance that earlier, shallower images had missed entirely.
“These galaxies look calm on the surface, but Webb allows us to see the subtle signs of past violence,” said Maltby.
Models match Webb’s findings
That combination of a smooth exterior and a disturbed interior is exactly what a recent, disruptive event would leave behind.
The galaxies’ extreme compactness fits the same story. Simulations of collisions between gas-rich galaxies tend to produce small, dense remnants.
Other simulations show that the tell-tale disturbance from such a crash fades within a few hundred million years. Catching it at all takes the sharp eyes of Webb.
Ancient galaxies leave clues
The disturbance does not appear in every quenched galaxy, and that pattern is telling.
It shows up in the massive systems of the early Universe – dense, round balls of stars known as red nuggets – but not in the smaller, later galaxies that the team also examined.
The massive red nuggets are round and spheroidal, about a quarter smaller than settled galaxies of the same weight. Now they carry hidden disturbance too. That pairing of compactness and disturbance is what violent mergers produce.
“The galaxies show clear signs of disturbance, telling us that something dramatic happened to them not long before their star formation shut down, most likely a merger with another galaxy,” said Maltby.
A separate study found that such collisions are common among post-starburst galaxies, and everything here lines up with that picture.
Smaller galaxies fade
The smaller, later galaxies tell a gentler story. These stay disc-shaped and carry no such hidden scars – a sign that milder processes, rather than a head-on smash, brought their star formation to an end.
One worry had hung over earlier work like this. Because dust and pockets of younger stars can distort how a galaxy looks, some researchers wondered whether the compactness was partly an illusion.
By measuring each galaxy at eight infrared wavelengths and finding that its form barely changed, the team ruled that out. The compactness is real, which means the earlier picture built from narrower data holds up.
Different paths to quenching
The upshot is a sharper account of how the young Universe’s giants died. A violent past lies beneath. The most likely trigger for the abrupt end of star formation in these massive early galaxies was a collision between galaxies.
Quenching, the work suggests, is not one process but at least two.
Massive galaxies in the early Universe seem to shut down through violent mergers, while smaller galaxies at later times fade more gently. The difference is now visible in how the two types are built.
Those dense early galaxies are thought to be the seeds of the giant elliptical galaxies that anchor the Universe today, so their deaths help explain the cosmos we see around us.
With deeper Webb observations, astronomers can now hunt for these mergers closer to the moment they happened and test how often a single collision does the job.
The study is published in Monthly Notices of the Royal Astronomical Society.
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