Astronomers discover oldest known star clusters in first Webb image

Astronomers just found a sparkling Easter egg in the first image taken by the James Webb Space Telescope: several 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 into a glittering sphere. And a team of astronomers studying Webb’s First Deep Field, a stunning image of the distant universe released July 12, says it has found five of the oldest globular clusters ever discovered, dancing like fireflies around a distant galaxy.

They published their results in The Astrophysical Journal Letters.

The Sparkler galaxy is one of thousands in Webb’s First Deep Field.NASA/ESA/Mowla et al. 2022

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

“We estimated the ages of the individual clusters from their relative luminosities at different wavelengths,” says Mowla and study co-author Kartheik Iyer, also an astrophysicist from the University of Toronto. Inverse. Older, cooler stars tend to emit most of their light at longer wavelengths, so the spectrum of light from a cluster of old 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 star clusters. NIRISS provided a key clue to the age of the clusters: oxygen.

Each chemical element absorbs or emits different wavelengths of light, so a detailed light spectrum can reveal what an object is made of. And the globular clusters of the Sparkler galaxy showed no trace of oxygen, which typically occurs in younger star clusters where new stars are still being born. This suggests that these five clusters were mostly filled with old, fading stars.

“It is difficult to resolve individual stars with our current data. “The only thing they have in common is that the population appears 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 about 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 “sparkly” globular clusters formed just a few hundred million years after the Big Bang.

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

Gravity Assist — The Hubble Space Telescope has imaged the Sparkler Galaxy before, but was unable to see the galaxy’s much smaller “flares,” which turned out to be incredibly ancient star clusters. At such a vast distance, even these bright balls of hundreds of thousands of stars would be too small for Webb to see — without the help of a nearby cluster of galaxies and a technique called gravitational lensing.

This diagram shows that gravitational lensing can magnify Webb’s view of distant objects.NASA/ESA/Mowla et al. 2022

The galaxy cluster SMACS 0723, which lies 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. It’s like what happens when light passes through a convex lens, but on a cosmic scale: a physical telescope magnified the Sparkler galaxy up to 100 times.

“Sparkler is a very cool result because it was an unusual combination of JWST’s incredible angular resolution (allowing us to see individual sparklers clearly) and Sparkler’s 10x to 100x magnification due to the gravitational lensing caused by the massive galaxy cluster in foreground. SMACS 0723),” say Mowla and Iyer. “Nor could it have triggered this discovery by itself.”

The gravitational lensing effect also helped Mowla and her colleagues confirm that the “flares” were really part of the Sparkler galaxy, not just bombarding it with flares.

“Our work is mainly focused on globular clusters near the edges of the visible part of the galaxy,” say Mowla and Iyer, which is why the sparklers of the Sparkler Galaxy appear to be floating around it.

In our own Milky Way galaxy, most of the 150 known globular clusters are located deep within the bulk of the galaxy itself. But for astronomers looking at a distant galaxy like the Sparkler Galaxy, the clusters at 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 will eventually break them apart.

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

“We need more observations to see if these apparently 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, being so far away can actually make that process easier. Most of the globular clusters that astronomers have studied so far are much closer to home, which also means they are part of the fairly recent universe. Astronomers looking at globular clusters in the Milky Way see light that left the stars somewhere between 12 billion and 13.5 billion years after the Big Bang. Because the universe is evolving more slowly than in the distant past, it can be difficult to determine an exact age for something so recent.

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

“This is important, as understanding whether globular clusters formed early when the first stars formed, versus later along with the rest of the galaxy, tells us a lot about how galaxies accumulated their stellar mass over time. says Mowla. and Iyer.

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

“Although they do not contain dark matter themselves, globular clusters are directly related to the individual dark matter haloes of the galaxies around which they orbit,” say Mowla and Iyer. For example, galaxies with more massive dark matter haloes also tend to have more globular clusters.

Because these twinkling balls of stars appear to form during a galaxy’s infancy, their relationship to dark matter could shed light on the relationship between dark matter and its final shape. 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 the help of five massive galaxy clusters and their gravitational lensing.

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

“Since the Sparkler is quite unique in what we’ve seen so far, we don’t really know if such galaxies are ubiquitous at high redshifts or if this is an isolated anomaly we happened to see, and the CANUCS data will shed light in it,” they say.

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

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