Massimo Pascale wasn’t planning to study the galaxy cluster SMACS 0723. But as soon as he saw the cluster glittering in the first image from the James Webb Space Telescope, or JWST, he and his colleagues couldn’t help themselves.
“We were like, we have to do something,” says Pascale, an astronomer at the University of California, Berkeley. “We can’t stop ourselves from analyzing this data. It was so exciting.”
Pascale’s team is one of several groups of scientists who saw the first JWST images and immediately rolled up their sleeves. In the first few days after images and the data used to create them were made public, scientists have estimated the amount of mass the cluster contains, uncovered a violent incident in the cluster’s recent past and estimated the ages of the stars in galaxies far beyond the cluster itself.
“We’ve been preparing for this for a long time. Myself, I’ve been preparing for years, and I’m not very old,” says Pascale, who is in his fourth year of graduate school. JWST “is really going to define a new generation of astronomers and a new generation of science as a whole.”
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Cluster collision
When the image of SMACS 0723 was released in a White House briefing on July 11, most of the focus went to extremely distant galaxies in the background (SN: 7/11/22). But smack in the middle of the image is SMACS 0723 itself, a much closer cluster of galaxies about 4.6 billion light-years from Earth. Its mass bends light from even farther away, making more distant objects appear magnified, as if their light had traveled through the lens of another cosmic-sized telescope.
The light from the most distant galaxy in this image started its journey to JWST about 13.3 billion years ago — “almost at the dawn of the universe,” says astrophysicist Guillaume Mahler of Durham University in England, who is already using the picture as his Zoom background.
But the image can also fill in the history of the intervening galaxy cluster itself. “People sometimes forget about that — the galaxy cluster is also very important,” Pascale says.
Pascale’s and Mahler’s teams each started by taking inventory of the distant galaxies that appear stretched and distorted in the image. The light from some of those galaxies is warped such that multiple images of the same galaxy appear in different places. Mapping those multiply imaged galaxies is a sensitive probe of the way mass is spread around the cluster. That, in turn, can reveal where the cluster contains dark matter, the invisible, mysterious substance that makes up the majority of the mass in the universe (SN: 9/10/20).
Both teams found that SMACS 0723 is more elongated than it appeared in previous observations. They also found a faint glow, called intracluster light, inside the cluster from stars that don’t belong to any particular galaxy. Together, those features suggest that SMACS 0723 is still recovering from a relatively recent smash-up with another galaxy cluster, the teams report separately in a pair of papers submitted to arXiv.org on July 14.
A galaxy cluster that has been sitting on its own for eons should have a rounder distribution of matter and intracluster light, rather than SMACS 0723’s oblong shape. The stars that emit the intracluster light were probably ripped from their home galaxies by gravitational forces during the collision.
“Two separate clusters have merged together, and it looks to us as if it’s not totally settled yet,” Pascale says. “What we might be looking at is an ongoing merger.”
Three examples of multiply imaged galaxies — marked with white, red and yellow arrows — popped out of this small region of the first JWST image. The gravity from a foreground galaxy cluster distorted the light from these galaxies, making them appear in at least two places at once.Reproduced from M. Pascale et al/arXiv.org 2022
Far-flung galaxies
Mapping out mass in the cluster is also essential to decoding the properties of the more distant galaxies in the background of the image, Mahler says. “You need to understand the cluster and its magnification power to understand what’s behind.”
Some scientists are already investigating those distant galaxies in detail. The first JWST data include not just pretty pictures but also spectra, measurements of how much light an object emits at various wavelengths. Spectra allow scientists to determine how much a distant object’s light has been stretched — or redshifted — by the expansion of the universe, which is a proxy for its distance. Such data can also help reveal a galaxy’s composition and the ages of its stars.
“The main thing that limits the study of star formation in galaxies is the quality of the data,” says astrophysicist Adam Carnall of the University of Edinburgh. But with the vastly improved data from JWST, he says, he and his team were able to measure the ages of stars in those remote galaxies.
Carnall and colleagues turned their attention to the spectra of the distant galaxies just a few days after the SMACS image was released. They measured the redshifts of 10 galaxies, five of which were particularly distant, the team reports in a paper submitted to arXiv.org on July 18. One had already been highlighted as the most distant galaxy ever seen, with light that was emitted just 500 million years after the Big Bang 13.8 billion years ago. The other four shone as late as 1.1 billion years after the Big Bang.
All 10 galaxies were relatively young when they emitted the light captured by JWST, Carnall says. They had all switched on their star formation just a few million years earlier. That’s not especially surprising, but it is interesting.
“The ability to look at these small, faint galaxies … gives you a sense of how all galaxies must look when they start forming stars,” Carnall says.
Scientists hope to use JWST to find the first instances of star formation ever. Other early results suggest they’re already getting close.
Some galaxies in a JWST image of another cluster may hearken from an even earlier time, as early as 300 million years after the Big Bang, two research teams report in a pair of papers submitted to arXiv.org on July 19. One of those galaxies seems to have already built up a spiral disk about a billion times the mass of the sun, which is surprisingly mature for such an early galaxy.
And a tally of galaxies seen in the SMACS 0723 image suggests that galaxies with mature disks, rather than disorganized blobs or ones made up mostly of dark matter, may have been more common in the very early universe than previously thought, another team reports in an arXiv.org paper submitted July 19. That means those early disks might not be outliers.
“Definitely these galaxies are a big deal, but it remains to be seen how exciting they will look in the context of a few months’ progress with JWST,” Carnall says. The best is yet to come.
