Staring off into the ancient past with a $10 billion space telescope, hoping to find extraordinarily faint signals from the earliest galaxies, might seem like a forlorn task. But it’s only forlorn if we don’t find any. Now that the James Webb Space Telescope has found those signals, the exercise has moved from forlorn to hopeful.
But only if astronomers can confirm the signals.
The James Webb Space Telescope (JWST) was built to peer back in time and identify the Universe’s very first galaxies. Those observations are meant to forge a link between the ancient galaxies and the galaxies we see now, including our own. That link will help astronomers understand how galaxies like ours formed and evolved over billions of years.
The expansion of the Universe stretches the light emitted by ancient objects billions of years ago. The stretching shifts the light toward the red end of the visible light spectrum. The James Webb Space Telescope was built to see this light and identify the ancient galaxies that emitted it.
The telescope’s GLASS Survey went to the heart of the issue. It used the galaxy cluster called Pandora’s Cluster (Abell 2744) as a gravitational lens to magnify distant galaxies behind it and found 19 bright objects that appear to be early galaxies.
Other early-release science results from the JWST found more objects that appear to be ancient galaxies. Together, these findings are a cornucopia of scientific observations. Astronomers set out decades ago to build the JWST with these findings in mind. But there’s a problem: our theories and models of galaxy formation suggest there shouldn’t be so many of these earliest galaxies. The JWST’s findings needed to be confirmed.
A team of researchers used the ESO’s ALMA (Atacama Large Millimetre/sub-millimetre Array) to examine a candidate galaxy from GLASS and to try to confirm it. Their paper is titled “Deep ALMA redshift search of a z ~ 12 GLASS-JWST galaxy candidate,” and it was published in the Monthly Notices of the Royal Astronomy Society. The lead author is Tom Bakx of Nagoya University.
Up until now, none of the JWST’s candidate ancient galaxies have been confirmed. Until astronomers can confirm them, we’re in a bind. In one of his Starts With A Bang articles at Big Think, astrophysicist Ethan Siegel made that point eloquently. “If all of these ultra-distant galaxy candidates were real, we’d have too many of them too early, forcing us to rethink how galaxies begin forming within the Universe,” Siegel writes. “But we might be fooling ourselves completely, and we won’t know for sure with only our current data. There’s a tremendous difference between the light that a distant galaxy emits and the light that arrives at our eyes after journeying for billions of light-years across the Universe.”
More observations were needed to confirm any of these ancient candidates, and that’s what this team of researchers gathered. “The first images of the James Webb Space Telescope revealed so many early galaxies that we felt we had to test its results using the best observatory on Earth,” lead author Bakx said in a press release.
They chose a galaxy named GHZ2/GLASS-z12, one of the brightest and most robust candidates at z > 10, according to the JWST observations. z > 10 means that the light from the galaxy has been travelling for over 13.184 billion years and has travelled a distance of at least 26.596 billion light-years. As Siegel pointed out in his article, a lot can happen to light that travels over 26 billion light-years before reaching us.
“Spectroscopy is needed to confirm the primeval nature of these candidates,” the authors write in their paper. It’s possible that the light from some of these galaxies is red due to dust rather than distance, and spectroscopy could help differentiate between the two. They turned to ALMA, the world’s most expensive ground-based telescope currently operating.
They used it to look for an oxygen line (O III) in the spectroscopy at the same frequency found in the JWST observations. O III is doubly-ionized oxygen, and it’s key because oxygen has a short formation time relative to other elements. Focusing on oxygen increased the likelihood of detection.
Stars can generate oxygen on a short 50 Myr time scale. Other elements, like carbon, for example, take nearly 500 Myr to appear in a galaxy. This means that oxygen is generally the best redshift indicator, according to the authors, and is likely the brightest emission line in the early Universe. Could ALMA find it?
ALMA’s power didn’t disappoint.
“The work of JWST has only just begun, but we are already adjusting our models of how galaxies form in the early Universe to match these observations.”
This figure from the study shows how the oxygen (O III) emission line (green contours) is shifted from the bright source the JWST detected (orange blur inside the yellow dotted line.) Image Credit: Bakx et al. 2023.ALMA’s confirmation wasn’t instantaneous, though. There was a slight shift in the oxygen signal between the JWST observations and the ALMA observation.
“We were initially concerned about the slight variation in position between the detected oxygen emission line and the galaxy seen by Webb,” author Tom Bakx notes, “but we performed detailed tests on the observations to confirm that this really is a robust detection, and it is very difficult to explain through any other interpretation.”
The observations do more than confirm the galaxy’s age, they also shed light on its metallicity. They show that enough stars had already lived and died by then to enrich the galaxy with elements like oxygen. “The bright line emission indicates that this galaxy has quickly enriched its gas reservoirs with elements heavier than hydrogen and helium. This gives us some clues about the formation and evolution of the first generation of stars and their lifetime,” said co-lead author Jorge Zavala of the National Astronomical Observatory of Japan.
The observations hold another tantalizing clue, too. At least some of the stars that lived and died and populated the galaxy with metals may have exploded as supernovae. “The small separation we see between the oxygen gas and the stars’ emission might also suggest that these early galaxies suffered from violent explosions that blew the gas away from the galaxy centre into the region surrounding the galaxy and even beyond,” added Zavala.
The image of galaxy GHZ2/GLASS-z12 with the associated ALMA spectrum. ALMA’s deep spectroscopic observations revealed a spectral emission line associated with ionized Oxygen near the galaxy, which has been shifted in its observed frequency due to the expansion of the Universe since the line was emitted.NASA / ESA / CSA / T. Treu, UCLA / NAOJ / T. Bakx, Nagoya U.
Finding the Universe’s earliest galaxies was a prime motivation behind the JWST, and as this study shows, it’s making progress. There are a growing number of candidate early galaxies awaiting confirmation, and if more of them are confirmed, as expected, astronomers will have their work cut out explaining them and updating their models of galaxy formation.
But that’s a good thing, according to Zavala. When scientists are forced to update their models due to new evidence, our understanding grows. This work shows how ALMA and the JWST can work in tandem to advance our knowledge. “We conclude that ALMA and JWST are highly synergistic, and together they should revolutionize our understanding of early galaxy formation and evolution,” the authors conclude in their paper.
“These deep ALMA observations provide robust evidence of the existence of galaxies within the first few hundred million years after the Big Bang and confirm the surprising results from the Webb observations,” Zavala said. “The work of JWST has only just begun, but we are already adjusting our models of how galaxies form in the early Universe to match these observations. The combined power of Webb and the radio telescope array ALMA gives us the confidence to push our cosmic horizons ever closer to the dawn of the Universe.”
