Astronomers discover oldest known star clusters in Webb's first image

Astronomers just found a sparkly Easter egg in the James Webb Space Telescope’s very first image: several teeming star clusters that may contain some of the oldest stars in the universe.

A globular cluster is a densely-packed group of thousands or even millions of stars, held together by their mutual gravity in a sparkling sphere. And a team of astronomers studying Webb’s First Deep Field, a stunning image of the distant universe released on July 12, say they’ve found five of the oldest globular clusters ever discovered, dancing like fireflies around a far-away galaxy.

They published their results in The Astrophysical Journal Letters.

What’s New — Amid the thousands of galaxies in Webb’s First Deep Field, astrophysicist Lamiya Mowla of the University of Toronto and her colleagues zoomed in on just one, 9 billion light years away and surrounded by small, sparkly yellow and red dots of light. They nicknamed it the “Sparkler Galaxy” and took a closer look at 12 of its “sparkles.” Five of them turned out to be the oldest and most distant globular clusters ever seen, home to stars that may have formed just half a billion years after the Big Bang.

“We estimated the ages of the individual clusters from their relative brightness at different wavelengths,” Mowla and study co-author Kartheik Iyer, also a University of Toronto astrophysicist, tell Inverse. Older, cooler stars tend to emit most of their light at longer wavelengths, so the spectrum of light from a cluster of elderly stars will be mostly reddish, compared to the bright blue glow of younger, hotter stars.

Mowla and her colleagues turned to another Webb instrument, the Near Infrared Imager and Slitless Spectrograph (NIRISS), for a more detailed look at the spectrum of light from the star clusters. NIRISS provided a key clue about the clusters’ age: oxygen.

Each chemical element absorbs or emits different wavelengths of light, so a detailed spectrum of light can reveal what an object is made of. And the Sparkler Galaxy’s globular clusters showed no trace of oxygen, which usually shows up in younger star clusters, where new stars are still being born. That suggested that these five clusters were mostly full of old, fading stars.

“It is difficult to resolve individual stars with our current data. The one thing they have in common is that the population seems to be quite old, with no signatures of young, massive stars that tend to have shorter lifetimes,” say Mowla and Iyer.

All the evidence so far suggests that the star clusters were around 4 billion years old when the light we see left them. And since the Sparkler Galaxy is about 9 billion light years away, that means the stars that make up its “sparkler” globular clusters formed just a few hundred million years after the Big Bang.

“These newly-identified clusters were formed close to the first time it was even possible to form stars,” said Mowla.

Gravity assist — The Hubble Space Telescope had imaged the Sparkler Galaxy before, but it couldn’t see the galaxy’s much smaller “sparklers,” which turned out to be incredibly ancient star clusters. At such a huge distance, even these bright balls of hundreds of thousands of stars would have been too small for Webb to see, either — without the help of a nearby cluster of galaxies and a technique called gravitational lensing.

Galaxy cluster SMACS 0723, which sits at the center of Webb’s First Deep Field, is so dense that its gravity actually bends spacetime. Light from objects on the far side of the galaxy cluster follows the curve of spacetime around it instead of shining straight through. It's like what happens when light passes through a curved lens, but on a cosmic scale: a natural telescope that magnified the Sparkler Galaxy up to 100 times.

“The Sparkler’s a really cool result because it was a serendipitous combination of JWST’s incredible angular resolution (that allows us to clearly see the individual sparkles), and 10x to 100x magnification of the Sparkler due to gravitational lensing caused by the massive foreground galaxy cluster (SMACS 0723),” say Mowla and Iyer. “Either on its own could not have enabled this discovery.”

The gravitational lens effect also helped Mowla and her colleagues confirm that the “sparklers” really were part of the Sparkler Galaxy, not just photobombing it.

“Our work does focus mainly on globular clusters near the edges of the visible part of the galaxy,” Mowla and Iyer say, which is why the Sparkler Galaxy’s “sparklers” seem to be hovering around it.

In our own Milky Way galaxy, most of the 150 known globular clusters are deep within the bulk of the galaxy itself. But for astronomers looking at a distant galaxy like the Sparkler Galaxy, clusters on the edges of the galaxy are much easier to see and study. They also tend to last longer than globular clusters in the inner region of a galaxy, where the pull of tidal forces would eventually rips them apart.

In fact, it’s a bit surprising to find massive globular clusters in such a distant — and therefore old — galaxy, precisely because the clusters don’t usually survive for so long.

“We need more observations to see if these seemingly long-lived massive globular clusters are an anomaly or if they redefine what we know about globular clusters at high redshifts,” say Mowla and Iyer.

Why It Matters — These ancient star clusters could eventually shed light on how galaxies form and even how dark matter behaves.

And ironically, the fact that they’re so far away may actually make that process easier. Most of the globular clusters astronomers have studied so far are much closer to home, which also means they’re part of the fairly recent universe. Astronomers looking at globular clusters in the Milky Way are seeing light that left the stars sometime between 12 billion and 13.5 billion years after the Big Bang. Because the universe is evolving more slowly than it did in the distant past, it can be hard to pinpoint an exact age for something so recent.

But in a much more distant galaxy like the Sparkler Galaxy, Mowla and her colleagues are seeing light from just 2 to 4 billion years after the Big Bang, and they say it’s much easier to guess the age of a star cluster from back when the universe was so young. Mowla compares it to trying a guess a person’s age based on their appearance: it’s easier to tell whether a child is 5 or 10 years old than it is to tell if an adult is 50 or 55.

“This is important, since understanding whether the globular clusters formed right at the beginning when the first stars were forming, versus later along with the rest of the galaxy, tells us a lot about how galaxies assembled their stellar mass over time,” say Mowla and Iyer.

Globular clusters from the very early universe, like the ones in the Sparkler Galaxy, could also help physicists piece together the still mysterious connection between dark matter and how galaxies form.

“Even though they don’t contain dark matter themselves, globular clusters are directly related to the individual dark matter halos of the galaxies they orbit around,” say Mowla and Iyer. For example, galaxies with more massive haloes of dark matter also tend to have more globular clusters.

Because these glittering balls of stars seem to form during a galaxy’s infancy, their relationship to dark matter could shed light on the relationship between dark matter and the eventual shape of the visible matter in a galaxy.

What’s Next — Next month, Mowla and her colleagues will use Webb’s NIRISS instrument to study five more deep fields, with help from five massive galaxy clusters and their gravitational lensing.

“We’re excited about the CANUCs galaxy clusters because they provide five additional fields to repeat this analysis in, and look for similar galaxies,” Mowla and Iyer say. That data could help astronomers understand whether the ancient globular clusters in the Sparkler Galaxy are a rare find, or something that’s common in the distant universe that we just couldn’t see before Webb.

“Since the Sparkler is pretty unique in what we’ve seen so far, we don’t actually know whether such galaxies are ubiquitous at high redshifts, or whether this is a one-off anomaly that we happened to see, and the CANUCS data will shed light on that,” they say.

Meanwhile, they hope to eventually use another Webb instrument, the Near Infrared Spectrograph (NIRSpec) to get a higher-resolution look at the spectrum of light from each of the Sparkler Galaxy’s globular clusters. That will help narrow down the clusters’ ages more precisely and reveal more about the types of stars in the clusters, their chemical makeup, and other properties.