Peering Back to the Early Universe: A Story of Confirmation and Refutation
NOIRLab Astronomers Use JWST to Observe the Universe's Earliest Galaxies and Uncover a Cunning ‘Mimic’
19 Sept. 2023
Today we see a Universe that is brimming with stars, galaxies, and supermassive black holes. But the very early Universe was likely a vastly different place. The familiar objects we find in the nearby Universe today may only loosely resemble their ancient forebears that existed when the Universe was a mere 500 million years old. These differences raise intriguing questions. How soon did galaxies form? What did they look like? When and how did the earliest stars and galaxies begin to light up the Universe?
Before 1999, astronomers were able to estimate the age of the Universe to be between 7 and 20 billion years. With advances in technology, we now know the Universe to be 13.8 billion years old, with an uncertainty of only 200 million years.
To help search for answers, NOIRLab astronomers Pablo Arrabal Haro, Mark Dickinson and their colleagues are probing the earliest galaxies and black holes in the cosmos thanks to NASA’s James Webb Space Telescope’s (JWST) and the Cosmic Evolution Early Release Science (CEERS) Survey. Their observations add support to growing evidence that the Universe was able to churn out remarkably luminous galaxies at surprisingly early epochs, as well as highlight the importance of follow-up observations to either confirm or refute outstanding objects.
With the advancement of technology, astronomers have been able to observe the Universe at farther and farther distances from our vantage point here on Earth. This also means they have been able to observe what the Universe looked like during earlier and earlier epochs. And over the past decade observations have continued to push back the estimated earliest times for the formation of galaxies. By just how much, though, can be difficult to determine.
Unexpectedly Bright and Abundant Early Galaxies
Last year astronomers analyzing early images from JWST identified a large number of surprisingly bright galaxies whose colors strongly suggested that they originate from far away and long ago when the Universe was less than 500 million years old. Additionally, these galaxies are unexpectedly luminous, suggesting that they formed stars earlier and more rapidly than astronomers had anticipated.
“If confirmed, CEERS-93316 would have set a new record for the earliest galaxy ever observed.”
During the first few months of JWST science in 2022, a few objects in particular caused a considerable buzz within the astronomy community. The CEERS team found several bright candidates, including one dubbed Maisie’s Galaxy, that was initially estimated to be seen at a time when the Universe was less than 400 million years old. Another team from the University of Edinburgh found an object, CEERS-93316, that appeared to be even more distant and more luminous — apparently only 240 million years after the Big Bang. With only images and colors from JWST, these objects remained candidates, but if confirmed CEERS-93316 would set a new record for the earliest galaxy ever observed and would seriously challenge our understanding of early cosmic evolution.
The astronomy community was buzzing with excitement about this potential record-setting discovery. But before astronomers could rejoice, proof of these galaxies’ extreme distances and young ages would require confirmation from JWST’s powerful infrared spectrograph NIRSpec, which would be able to perform measurements of each galaxy’s light in order to precisely determine their distances. And that’s just what Arrabal Haro and Dickinson set out to do.
In a new paper, led by Arrabal Haro and published in the journal Nature, the CEERS and Edinburgh teams joined forces to observe several of the previously discovered bright, early galaxy candidates with NIRSpec. By observing the candidates in a range of different wavelengths, the team is able to see at what wavelength each galaxy is no longer visible, which then allows them to deduce their cosmological age. With these observations both Maisie’s Galaxy and another galaxy, the more prosaically named CEERS2_588, were confirmed to indeed be remarkably young and bright — seen only 390 and 410 million years after the Big Bang, respectively, when the Universe was less than 3% of its current age.
"What distinguishes Maisie’s Galaxy and CEERS2_588 from the few other confirmed galaxies at that epoch of the Universe is that they are more luminous,” said Arrabal Haro. “Confirming two ultra-distant, very luminous galaxies reinforces the idea of a fast growth of galaxies in the first few hundred million years after the Big Bang, which is hard to explain with our current models of galaxy formation."
These same crucial spectroscopic measurements, however, exposed the contender for the earliest galaxy ever observed, CEERS-93316, as a cunning mimic. Though previously thought to have formed when the Universe was only 240 million years old, it is in fact seen much later, when the Universe was 1.2 billion years old. An unusual coincidence of strong emission from hydrogen and oxygen, heavy obscuration by dust, and the galaxy’s very specific distance all conspire to impersonate the colors of a far more distant, earlier object, fooling the teams that analyzed the earlier CEERS data.
“Extraordinary candidates require conclusive proof.”
“You win some, you lose some!”, observed Dickinson. “On the one hand, we have verified that there was an abundant population of surprisingly luminous galaxies at very early times, forming stars more rapidly than we had anticipated. At the same time, CEERS-93316 offers a cautionary tale: extraordinary candidates require conclusive proof.”
The CEERS team has now observed more than a thousand faint galaxies with NIRSpec. In three papers (two led by Arrabal Haro from NOIRLab, and one led by Seiji Fujimoto of the University of Texas at Austin), they have confirmed 16 galaxies at cosmic ages between 390 and 640 million years after the Big Bang. These results not only propel astronomers’ understanding of the early Universe forward by reinforcing the higher-than-expected abundance of bright galaxies, but also highlight the importance of follow-up observations in order to confirm or refute potential ground-breaking discoveries.
“Without more information, galaxies’ colors can trick us,” said Arrabal Haro, “but JWST’s NIRSpec spectroscopy is an extraordinary tool both for confirming and refuting the remarkable candidates that JWST is discovering.”