Only a few hundred million years after the Big Bang, the infant Universe was brimming with opaque hydrogen gas that trapped light at some wavelengths from stars and galaxies. Over the first billion years, the gas became fully transparent — allowing the light to travel freely. Astrophysicists have long sought definitive evidence to explain this flip. The new data from the NASA/ESA/CSA James Webb Space Telescope recently pinpointed the answer using a set of galaxies that existed when the Universe was only 900 million years old. Stars in these galaxies emitted enough light to ionize and heat the gas around them, forming huge, transparent bubbles; eventually, those bubbles met and merged, leading to today’s clear and expansive views.
Webb captured extraordinarily detailed images and spectra of galaxies that existed when the Universe was only 900 million years old. Image credit: NASA / ESA / CSA / Simon Lilly, ETH Zurich / Daichi Kashino, Nagoya University / Jorryt Matthee, ETH Zurich / Christina Eilers, MIT / Rongmon Bordoloi, NCSU / Ruari Mackenzie, ETH Zurich / Alyssa Pagan, STScI / Ruari Mackenzie, ETH Zurich.
“Not only does Webb clearly show that these transparent regions are found around galaxies, we’ve also measured how large they are,” said Nagoya University astronomer Daichi Kashino.
“With Webb’s data, we are seeing galaxies reionize the gas around them.”
“These regions of transparent gas are gigantic compared to the galaxies — imagine a hot air balloon with a pea suspended inside.”
“The Webb data show that these relatively tiny galaxies drove reionization, clearing massive regions of space around them.”
“Over the next hundred million years, these transparent bubbles continued to grow larger and larger, eventually merging and causing the entire Universe to become transparent.”
Kashino and colleagues targeted a time just before the end of the Epoch of Reionization, when the Universe was not quite clear and not quite opaque — it contained a patchwork of gas in various states.
They focused Webb on the quasar SDSS J0100+2802, an extremely luminous active supermassive black hole that acts like an enormous flashlight, highlighting the gas between the quasar and the telescope.
As the quasar’s light traveled toward us through different patches of gas, it was either absorbed by gas that was opaque or moved freely through transparent gas.
“By illuminating gas along our line of sight, the quasar gives us extensive information about the composition and state of the gas,” said MIT astronomer Anna-Christina Eilers.
The astronomers then used Webb to identify galaxies near this line of sight and showed that the galaxies are generally surrounded by transparent regions about 2 million light-years in radius.
In other words, Webb witnessed galaxies in the process of clearing the space around them at the end of the Epoch of Reionization.
To put this in perspective, the area these galaxies have cleared is approximately the same distance as the space between our Milky Way Galaxy and the Andromeda galaxy.
Until now, astronomers didn’t have this definitive evidence of what caused reionization — before Webb, they weren’t certain precisely what was responsible.
“What do these galaxies look like? They are more chaotic than those in the nearby Universe,” said ETH Zürich astronomer Jorryt Matthee.
“Webb shows they were actively forming stars and must have been shooting off many supernovae. They had quite an adventurous youth!”
“Along the way, we used the Webb data to confirm that the black hole in the quasar at the center of this field is the most massive currently known in the early Universe, weighing 10 billion times the mass of the Sun.”
“We still can’t explain how quasars were able to grow so large so early in the history of the Universe. That’s another puzzle to solve.”
The exquisite Webb images also revealed no evidence that the light from the quasar had been gravitationally lensed, ensuring that the mass measurements are definitive.
The team will soon dive into research about galaxies in five additional fields, each anchored by a central quasar.
Webb’s results from the first field were so overwhelmingly clear that they couldn’t wait to share them.
“We expected to identify a few dozen galaxies that existed during the Epoch of Reionization — but were easily able to pick out 117. Webb has exceeded our expectations,” Dr. Kashino said.
The findings appear in the Astrophysical Journal.
