For two years, an international team has been exploring what astronomers refer to as the Cosmic Dawn—the period in the first few hundred million years following the Big Bang where the first galaxies were born.
Using NASA’s James Webb Space Telescope (JWST), they’ve now discovered two of the earliest and most distant galaxies yet confirmed.
Dating back to just 300 million years after the Big Bang, these galaxies mark a major milestone in the study of the early universe, according to University of California-Santa Cruz astronomer Brant Robertson, who co-led the team working in JADES (the JWST Advanced Deep Extragalactic Survey).
“This discovery is completely unanticipated and is likely to be seen as the most significant extragalactic discovery with JWST to date,” said Robertson, an astronomy and astrophysics professor who sits on the JADES steering committee. He is the lead author on the first of three papers reporting various aspects of the discovery.
In one paper, the authors concluded: “With high-redshift galaxy populations now established fewer than 300 million years after the Big Bang, we have extended our reach into the cosmic past by 40% during the first eighteen months of JWST operations.”
“Redshift” refers to an effect caused by the expansion of the universe, where the wavelength of light from distant galaxies stretches as it travels. In these newly discovered galaxies, the effect is extreme—stretching by a factor of 15, and moving even the ultraviolet light of the galaxies to infrared wavelengths where only JWST has the capability to see it.
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Chasing early galaxies
Modern theory holds that galaxies develop in special regions where gravity has concentrated cosmic gas and dark matter into dense lumps known as “halos.” These halos evolve quickly in the early universe, merging into more and more massive collections of matter. This rapid development is why astronomers are so eager to find yet earlier galaxies: Each small increment moves our eyes to a less developed period, where brighter galaxies are even more distinctive and unusual.
“This galaxy is truly a gem, and it points at more hidden treasures in the early universe,” said Professor Robertson.
Found in a region near the Hubble Ultra Deep Field, the new galaxies, which have been confirmed spectroscopically, are now known as JADES-GS-z14-0 (the more distant one) and JADES-GS-z14-1.
According to NASA, in addition to being the new distance record holder, JADES-GS-z14-0 is remarkable for how big and bright it is. JWST measures the galaxy at over 1,600 light-years in diameter. Many of the most luminous galaxies produce the bulk of their light via gas falling into a supermassive black hole, producing a quasar. But the team says the large size of JADES-GS-z14-0 means that the light must be produced by young stars.
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Deeply hidden gems
And yet, the massive galaxy was a puzzle for the JADES team when they first spotted it over a year ago, as it appears close enough on the sky to a foreground galaxy that they couldn’t be sure that the two weren’t neighbors. But in October 2023, the JADES team conducted even deeper imaging—five full days with the JWST Near-Infrared Camera on just one field—to form the “JADES Origins Field.” With the use of filters designed to better isolate early galaxies, confidence grew that JADES-GS-z14-0 was indeed very distant.
In addition, the galaxy happened to fall in a region where the team had conducted ultra-deep imaging with the JWST Mid-InfraRed Instrument. These combined imaging results convinced the team to include the galaxy in what was planned to be the capstone observation of JADES, a 75-hour campaign to conduct spectroscopy on faint early galaxies. The spectroscopy confirmed their hopes that JADES-GS-z14-0 was indeed a record-breaking galaxy—and that the fainter candidate, JADES-GS-z14-1, was nearly as far away.
The combination of the high luminosity and the stellar origin makes JADES-GS-z14-0 the most distinctive evidence yet for the rapid formation of large, massive galaxies in the early universe.
“We could have detected this galaxy even if it were 10 times fainter, which means that we could see other examples yet earlier in the universe—probably into the first 200 million years,” Robertson added.
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Robertson’s May 30 paper, Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic Star-Formation Rate Density 300 Myr after the Big Bang, is accepted for publication in the Astrophysical Journal.
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