When a bunch of photons hit the nearly flawless mirrors of the James Webb Space Telescope earlier this year, they had been traveling through vacuum 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 a cosmic sense, recent history. Ancient doesn’t really do it justice.
Webb’s first deep-field images – infrared recordings of tiny chunks of sky filled with galaxies –sparked a stampede among astronomers to find the oldest galaxies in sight. The Hubble Space Telescope held the existing record with observations of a galaxy dating back to when the universe was just 400 million years old. Webb’s larger mirrors and his ability to see into the infrared parts of the spectrum were designed to do better.
The telescope proved its worth on Friday when a team of scientists – known collectively as JADES, a collaboration between the builders of two of Webb’s instruments, NIRcam and NIRspec – announced that they had confirmed observations of the galaxies oldest to date.
“For the first time, we discovered galaxies only 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 published, stories of extremely old galaxies hit the web.
But while NIRcam observations revealed a rich population of targets worthy of further investigation, official confirmation required detailed spectroscopic analysis.
“It’s very possible that closer galaxies are masquerading as very distant galaxies,” said astronomer and co-author Emma Curtis-Lake of the University of Hertfordshire in the UK.
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. Although neither study has yet been peer-reviewed, the results likely broke Hubble’s record.
The observed sliver of sky is about the size of the Queen’s eye “on a one-pound coin held at arm’s length”, said Renske Smit of Liverpool John Moores University. say it BBC. Within this 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, drawing it into the redder parts of the spectrum. Some of the oldest light has been stretched out of the visible spectrum and into the infrared, Webb’s specialty.
The oldest galaxies are not only visible in the infrared, but their spectra also intersect at a precise point due to intergalactic hydrogen scattering. Faint infrared galaxies exhibiting this cutoff, which moves with a greater redshift, have filled a pool of candidates. The team then spent 28 hours observing 250 of them with NIRspec. This detailed spectroscopic analysis included specific atomic signatures and identified the redshift.
Four galaxies were found to be exceptionally old, with redshifts greater than 10. Two showed redshifts at 13 years old, at 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. The Milky Way, by comparison, is said to have at least 100 billion stars and the sun is around 4.6 billion years old. Despite their small size, the team says these early galaxies churned out stars at a prodigious rate, up to 10 times faster than nearby similar-sized galaxies in the present day.
These galaxies now appear to hold the record for the oldest ever confirmed by spectroscopy, but the title may not last long. Although still awaiting confirmation, scientists have estimated that some galaxies already captured by Webb are even older, and Webb was designed to see light from epochs as early as 100 million years after the Big Bang.
By studying the first stars and galaxies, scientists I hope to know more on the formation of galaxies and to identify a period in the evolution of the universe known as reionization, when the strong light of the first stars ionized the surrounding gas by removing electrons from hydrogen and helium. As stars in these four galaxies may have started forming up to 100 million years earlier, this first generation of stars could date back to around 230 million years after the Big Bang.
“With these measurements, we can know the intrinsic luminosity of galaxies and determine how many stars they have,” Robertson said. “Now we can start to really distinguish how galaxies are assembled over time.”
Image Credit: NASA, ESA, CSA and STScI