An international team of astronomers has discovered the oldest and most distant galaxies confirmed to date using data from the James Webb Space Telescope (JWST). The telescope captured the light emitted by these galaxies more than 13.4 billion years ago, which means that the galaxies date from less than 400 million years after the Big Bang, while the universe was only 2% of its current age. .
Initial JWST observations threw up several candidate galaxies at extreme distances, just like previous observations with the Hubble Space Telescope. Now four of these targets have been confirmed by obtaining long spectroscopic observations, which not only provide reliable measurements of their distances, but also allow astronomers to characterize the physical properties of galaxies.
“We discovered galaxies at incredibly early times in the distant universe,” said Brant Robertson, a professor of astronomy and astrophysics at UC Santa Cruz. “With JWST, for the first time, we can now find galaxies that far away and then confirm via spectroscopy that they really are that far away. »
Astronomers measure the distance to a galaxy by determining its redshift. Due to the expansion of the universe, distant objects appear to recede from us and their light is stretched to longer, redder wavelengths by the Doppler effect. Photometric techniques based on images captured through various filters can provide redshift estimates, but definitive measurements require spectroscopy, which separates light from an object into its component wavelengths.
The new findings focus on four galaxies with redshifts greater than 10. Two galaxies originally observed by Hubble have now confirmed redshifts of 10.38 and 11.58. The two most distant galaxies, both detected in JWST images, have redshifts of 13.20 and 12.63, making them the most distant galaxies confirmed by spectroscopy to date. A redshift of 13.2 corresponds to about 13.5 billion years.
“These go way beyond what we could have imagined finding before JWST,” Robertson said. “At redshift 13, the universe is only about 325 million years old. »
Robertson and Emma Curtis-Lake of the University of Hertfordshire (UK) will present the new findings on December 12 at a conference at the Space Telescope Science Institute (STScI) in Baltimore on “First Science Results from JWST.” They are the lead authors of two papers on the findings that have not yet gone through the peer review process.
The observations are the result of a collaboration of scientists who led the development of two of Webb’s onboard instruments, the Near Infrared Camera (NIRCam) and the Near Infrared Spectrograph (NIRSpec). The study of the faintest and oldest galaxies was the main motivation for the concepts of these instruments. In 2015, the instrument teams came together to propose the JWST Advanced Deep Extragalactic Survey (JADES), an ambitious program allotted just over a month of telescope time and designed to provide a view of the primordial universe. unprecedented in depth and detail. . JADES is an international collaboration of more than eighty astronomers from ten countries.
“These results are the culmination of why the NIRCam and NIRSpec teams have partnered to run this observing program,” said Marcia Rieke, NIRCam Principal Investigator at the University of Arizona.
The JADES program began with NIRCam, using more than 10 days of mission time to observe a small patch of sky in and around the Hubble Ultra Deep Field. Astronomers have studied this region for more than 20 years with almost all major telescopes. The JADES team observed the field in nine different infrared wavelength ranges, capturing exquisite images that reveal nearly 100,000 distant galaxies, each billions of light-years away.
The team then used the NIRSpec spectrograph over a single three-day observing period to collect light from 250 faint galaxies. This gave precise measurements of the redshift and revealed the properties of the gas and stars in these galaxies.
“With these measurements, we can learn about the intrinsic luminosity of galaxies and determine how many stars they have,” Robertson said. “Now we can start to really make out how galaxies assemble over time. »
Co-author Sandro Tacchella from the University of Cambridge in the UK added: “It is difficult to understand galaxies without understanding the initial periods of their development. As with humans, much of what happens next depends on the impact of these first generations of galaxies. So many questions about the galaxies have been waiting for Webb’s transformative opportunity, and we’re thrilled to be able to participate in telling this story. »
According to Robertson, star formation in these early galaxies began about 100 million years before the age at which they were observed, pushing the formation of the first stars back to about 225 million years after the Big Bang.
“We are seeing evidence of star formation as soon as expected based on our models of galaxy formation,” he said.
Candidate galaxies for even larger redshifts have been identified by other teams based on photometric analyzes of JWST images, but these have yet to be confirmed using spectroscopy. JADES will continue in 2023 with a detailed study of another field, this one focused on Hubble’s iconic deep field, then return to the ultra-deep field for another round of deep imaging and spectroscopy. Many other candidates in the field await spectroscopic research, with hundreds of additional hours already approved.
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