The 'Fireflies' of the Early Cosmos

Astronomers have detected luminous clusters of galaxies much earlier in the universe’s history than expected.

Artist's impression of luminous galaxies tightly packed into one region of space in the early universe
Artist's impression of luminous galaxies tightly packed into one region of space in the early universe (ESO / M. Kornmesser)

Close to the beginning of time, not long after the Big Bang, cosmic construction was booming. Giant clumps of gas collapsed under their own weight and formed the first stars. Eventually, galaxies emerged, tugged into glittering arrangements by gravity. Some of them clustered together.

This process took many millions of years to unfold, but thanks to some very powerful telescopes, we now have a mesmerizing snapshot of the events.

Astronomers have detected the light coming from two clusters rich with individual galaxies. Their observations show the two clusters as they were when the universe was just 1.5 billion years old, about one-tenth of its current age.

The clusters were first spotted by the South Pole Telescope, which is located in Antarctica, and the Herschel Space Observatory, a now-defunct spacecraft that orbits nearly 1 million miles from Earth. To these telescopes, each cluster looked like a speck of light against a backdrop of darkness. So astronomers used two ground-based telescopes in the Chilean desert to get a closer look. These observations were able to distinguish brightly glowing galaxies within the clusters, revealing 10 in one and 14 in the other. The discoveries, made by two teams of international scientists, are described in papers published Wednesday in Nature and The Astrophysical Journal.

Here’s a look at the cluster that was found to contain 14 galaxies, through the eyes of the South Pole Telescope and the two Chilean telescopes, APEX and ALMA:

ALMA / ESO / NAOJ / NRAO / Miller et al.

The images may not seem like much at first glance—look, just more specks of light!—but consider that these specks, all the tiny particles of light called photons, spent billions of years traversing the vastness of the cosmos before finally reaching Earth and then, a comparatively short time later, the screen in front of your face.

The detection of these galaxy groupings surprised astronomers, who have long thought that such young clusters, or protoclusters, took much longer to form in the early universe. They expected them to start showing up about 3 billion years after the Big Bang. How these baby clusters got as big as they did this fast is a mystery.

So is why the individual galaxies in the clusters all seem to be in the same stage of development. These galaxies, known as starburst galaxies, are star-making factories, churning out new stars at an incredibly rapid rate. Our Milky Way galaxy produces about one star a year, according to astronomers. Starburst galaxies, as their name so wonderfully suggests, can produce thousands in the same amount of time, and they shine with tremendous intensity.

Scientists had expected to find a mix of galaxies in the early universe, some young and some slightly less young, some luminous and others a little dimmer. “To find this whole population just all lit up is pretty unusual,” says Michael West, an astronomer at the Lowell Observatory in Arizona who studies galaxy formation, and who was not involved in these studies.

West likens the surprising discovery to stumbling upon a gaggle of fireflies in your backyard on a summer night, all glowing in unison. “If you go outside at night, you’ll see the occasional firefly here and there,” he says. “But imagine if you were out at night and you saw suddenly 30 fireflies, all together in this little region of space, all light up at the same time. That’s kind of what we’re seeing here.”

Starburst galaxies are short-lived creatures. Stars are born from the collapse of clouds of gas and dust, and galaxies eventually start running low on their supply of these pockets of creation. Star formation slows down and galaxies begin to dim.

Astronomers don’t know the mechanics behind the starburst galaxies in these clusters. Perhaps the galaxies were born simultaneously out of the same, highly concentrated region of gas and dust, or they formed nearby and the forces of gravity drew them closer. It could be that their close proximity to each other is fueling even more star formation than if they were floating solo. “With so many galaxies packed into such a small volume of space—small in a relative sense—they’ll have lots of mergers going on and they’ll build one or more big galaxies,” West says. And galaxy mergers are great for making new stars out of gas clouds.

“There’s this constant battle between gravity, which would love to shrink [gas clouds], and external heat and pressure, which tries to prevent that from happening,” West says. “And when galaxies collide, the gas gets squeezed and shocked, and that triggers the birth of more stars.”

Such cosmic collisions are less violent than they sound. Mergers can last for many millions of years, and the stellar inhabitants of the galaxies will remain untouched. “The odds of any two stars hitting each other is essentially zero,” West says. “If you got in your car and you drove to the sun, it would take you about 170 years, if you drove 60 miles an hour and don’t stop to pee or anything. If you wanted to drive to the nearest star to us, Alpha Centauri, it would take you 50 million years—and that’s our nearest neighbor.”

The discovery of the baby clusters means that astronomers must, once again, rethink their predictions for galaxy formation, a process they’re still trying to pin down. But that’s simply the nature of this kind of research. The deeper telescope technology stretches our gaze into the universe, illuminating the spaces shrouded in darkness, the more surprises we have found.