Astronomers have just confirmed the oldest galaxies ever observed
When a group of photons struck the near-flawless mirrors of the James Webb Space Telescope earlier this year, they had traveled through the void for 13.4 billion years. The light was emitted from distant galaxies at a time when the birth of everything we know and see was still in the past in a cosmic sense. Ancient doesn’t really do it justice.
Webb’s first deepfield images – infrared images of tiny patches of sky, teeming with galaxies –led to a struggle among astronomers to find the oldest galaxies in view. The Hubble Space Telescope held the existing record with observations of a galaxy from a time when the universe was only 400 million years old. Webb’s larger mirrors and ability to look into the infrared parts of the spectrum are designed to do better.
On Friday, the telescope proved its mettle when a team of scientists — collectively known as JADES, a collaboration between the builders of two of Webb’s instruments, NIRcam and NIRspec — announced that they had confirmed observations of the oldest galaxies yet.
“For the first time, we only discovered galaxies 350 million years after the Big Bang, and we can be absolutely sure of their fantastic distances,” said Brant Robertson of the University of California Santa Cruz, a member of the NIRCam science team and co-author of a recent paper on the work.
Astronomers first began compiling a list of candidates by analyzing data from Webb’s NIRcam instrument, a highly sensitive infrared camera. Almost immediately after Webb’s first images were made public, stories about extremely old galaxies began to appear on the web.
But while NIRcam observations revealed a rich population of targets worthy of closer scrutiny, official confirmation required detailed spectroscopic analysis.
“It’s quite possible that nearby galaxies are masquerading as very distant galaxies,” said astronomer and study co-author Emma Curtis-Lake of the University of Hertfordshire in the United Kingdom.
Thanks to NIRspec, in two recent studies (here and here), the teams were able to perform spectroscopic analysis – the gold standard for confirming the distance and age of these incredibly faint early galaxies – for a range of candidates. While neither study has yet been peer-reviewed, the findings likely beat Hubble’s record.
The observed patch of sky is about the size of the queen’s eye “on a pound coin held at arm’s length”, Renske Smit, Liverpool John Moores University told the BBC. Within that eye are nearly 100,000 galaxies, each captured at some point billions of years ago.
To measure the age of a galaxy near the beginning of the universe, scientists measure its “redshift.” As light travels, the expansion of the universe extends its wavelength and pulls it toward the redder parts of the spectrum. Some of the oldest light has stretched from the visible spectrum to the infrared – Webb’s specialty.
The oldest galaxies are not only visible in the infrared, but their spectrum is also cut off at some point by the scattering of intergalactic hydrogen. Faint infrared galaxies showing this clipping, which moves with greater redshift, filled a pool of candidates. The team then spent 28 hours observing 250 of these with NIRspec. This detailed spectroscopic analysis included specific atomic signatures and captured the redshift.
Four galaxies were found to be exceptionally old, with redshifts greater than 10. Two showed redshifts at 13, from a time when the universe was only 330 million years old. The team says these galaxies are small, only a hundred million solar masses, and made up of young stars less than 100 million years old. For comparison, the Milky Way is said to have at least 100 billion stars and the sun is about 4.6 billion years old. Despite their diminutive size, the team says these early galaxies produced stars at a prodigious rate, as much as 10 times faster than similarly sized galaxies closer to the present time.
These galaxies now appear to hold the record for oldest ever spectroscopically confirmed, but the title may not last long. While still awaiting confirmation, scientists estimate that some galaxies already captured by Webb are even older, and Webb is designed to see light from epochs as early as 100 million years after the Big Bang.
By studying the earliest stars and galaxies, scientists hope to learn more about the formation of galaxies and to establish a period in the evolution of the universe known as reionization, when the strong light from the first stars ionized surrounding gas by stripping electrons from hydrogen and helium. Since the stars in these four galaxies may have formed as much as 100 million years earlier, this first generation of stars could date from about 230 million years after the Big Bang.
“With these measurements, we can know the intrinsic brightness of the galaxies and figure out how many stars they have,” Robertson said. “Now we can really start to take apart how galaxies are put together over time.”
Image credit: NASA, ESA, CSA and STScI
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