Cosmic Fog Lifted Earlier Than Expected

An international team of astronomers using the James Webb Space Telescope (JWST) has shed new light on one of the most important events in the early history of our universe. According to detailed observations of a tiny, remote galaxy, the Epoch of Reionization started much earlier than most cosmologists had thought possible — a mere 330 million years after the Big Bang.

“This result was totally unexpected by theories of early galaxy formation and has caught astronomers by surprise,” says team member Roberto Maiolino (University of Cambridge, UK) in a NASA/ESA press release.

Just 380,000 years after its fiery beginnings, the expanding universe turned into a dark fog of mostly neutral hydrogen and helium atoms, entering the so-called cosmic dark ages. Only after the first massive stars and fledgling galaxies were born, a period known as Cosmic Dawn, did high-energy radiation start to slowly lift the fog as it ionized the tenuous intergalactic gas — ripping nuclei and electrons apart into a plasma of charged particles. This period is called the Epoch of Reionization. (It is called re-ionization because cosmic matter also started out in an ionized plasma state immediately after the Big Bang.)

During the Epoch of Reionization, the idea goes, huge bubbles of ionized gas started to form around young galaxies. Over time, these bubbles grew larger and began to overlap, until all of intergalactic space became ionized. To better understand the physics of the early universe as well as the origin and evolution of the first galaxies, cosmologists want to know when and how fast this process took place.

In a paper in today’s Nature, astronomers present JWST observations of a very small galaxy that is so distant that the light we see today was emitted when the universe was just 330 million years old. “Even to JWST, it’s a point source,” says study lead Joris Witstok (Niels Bohr Institute, Denmark), “implying that it cannot be larger than some 70 parsecs [230 light-years] across.” Yet, the galaxy, known as JADES-GS-z13-1-LA, somehow produces so much high-energy radiation that many hydrogen atoms in the local interstellar gas are being ionized, as evidenced by JWST spectroscopy.

Using the space telescope’s sensitive Near Infrared Spectrograph for 19 hours on end, the team observed the galaxy’s Lyman-alpha emission. This light is emitted at an ultraviolet wavelength of 121.5 nanometers, but during their 13.5-billion-year trip through expanding space to Earth, the light waves are stretched to the much-longer near-infrared wavelength of 1.7 microns. (In technical terms, the galaxy’s redshift is 13.05.)

Lyman-α emission is produced when hydrogen nuclei and electrons recombine into neutral atoms, which frequently occurs in the relatively dense interstellar environment of a galaxy. But for recombination to occur, the gas has to be ionized in the first place, so the detection of strong Lyman-α emission points to the presence of huge amounts of high-energy, ionizing radiation.

Unfortunately, the JWST observations cannot yet reveal anything conclusive about what the source of this radiation is. Incredibly massive and luminous stars could generate intense ultraviolet light, but so could a black hole feeding on gas in the core of the diminutive galaxy. In the case of stars, they would need to be much more extreme than described by current stellar models, says Witstok. In the case of an active nucleus, it would be the earliest accreting supermassive black hole known to date.

Although the source of the radiation remains unknown, the fact that the Ly-α line is visible at all in the spectrum of JADES-GS-z13-1-LA implies that the Epoch of Reionization had already begun when the universe was 330 million years old. If there were no bubble of tenuous ionized intergalactic gas surrounding the galaxy, the Lyman-α photons would have been absorbed again by intervening neutral hydrogen atoms. The authors estimate that the bubble must have a radius of at least 650,000 light-years.

According to Michele Trenti (University of Melbourne), the new observations are “both surprising and exciting.” Until now, most cosmologists believed the onset of the reionization era occurred much later, somewhere between 500 and 700 million years after the Big Bang. The new result “suggests that early-forming galaxies are more efficient than previously thought at reheating the universe,” Trenti says.

Team member Kevin Hainline (University of Arizona) agrees. In the press release accompanying the Nature paper, he says “We really shouldn’t have found a galaxy like this, given our understanding of the way the universe has evolved.”

It’s certainly not the first time that JWST is revealing surprising news about the very early universe. In an accompanying News & Views perspective piece in the same issue of Nature, Trenti observes: “This Lyman-α detection contributes another piece of the puzzle, strengthening the feeling that astronomers are missing at least one key component for understanding how gas is converted into stars at the earliest times, or how it becomes accreted onto seeds that later become supermassive black holes.”

Meanwhile, Witstok and his colleagues keep combing through JWST data to find more remote Lyman-α-emitting galaxies. “They are needles in a haystack,” he says, “but we expect to find others within the next year or so.”