Astronomers have spotted a laser-like beam of microwaves produced by two galaxies smashing together, which is the brightest and most distant example of this phenomenon ever seen.

To produce a laser, first atoms need to be stimulated into an unstable, higher-energy state. Then particles of light, or photons, fired at these atoms will cause them to relax and emit their own photons, causing a chain reaction that produces many more photons in the process. Because each atom emits identical photons, all of the light being produced is at the same frequency, forming a beam of coherent light.

The same process can happen when galaxies smash together. Gas from both galaxies gets compressed, producing more stars and light. After travelling through clouds of dust, this light can then excite hydroxyl ions, which consist of hydrogen and oxygen atoms, into higher energy states. When these excited ions are blasted with radio waves, such as from a supermassive black hole, they can suddenly relax and produce a beam of extremely bright and focused microwave radiation, known as a maser.

Now, Roger Deane at the University of Pretoria in South Africa and his colleagues have spotted the brightest and most distant maser so far, in a galaxy nearly 8 billion light years away called H1429-0028. The light from this galaxy is warped by a massive galaxy between it and Earth that acts as a magnifying glass, an effect called gravitational lensing.

Deane and his colleagues were using the MeerKAT telescope in South Africa, which consists of 64 linked radio telescopes that act as one giant dish, to look for galaxies rich in molecular hydrogen, which emit light at a telltale frequency. But when they turned MeerKAT towards H1429-0028, they saw light being strongly emitted at a higher frequency, which they knew was only produced by powerful masers.

“We had a quick look at the 1667 megahertz [frequency], just to see whether it was even detectable, and there was this booming, huge [signal]. It was immediately the record,” says Deane. “It was serendipitous.”

The light beam from the galaxy is so bright that the maser may warrant its own category, called a gigamaser, much more powerful than the megamasers that have been observed in galaxies closer to us. “This is about 100,000 times the luminosity of a star, but in a distant galaxy, concentrated into a very, very small part of the [electromagnetic] spectrum,” says Deane.

We should be able to detect similar masers at much greater distances when the Square Kilometre Array in South Africa, a much larger and more sensitive version of MeerKAT, is completed and comes online in the coming years, says Matt Jarvis at the University of Oxford.

Such distant galaxy masers will be from some of the first galaxies formed in the universe and could give us precise information about how galaxies were merging far back in time, says Jarvis. “[Masers] need very precise conditions,” he says. “You need this radio continuum emission and you need this infrared emission, which you only really get from dust heated around forming stars. In order to get these very specific physical conditions to get the maser in the first place, you need merging galaxies.”

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