James Webb discovers earliest sign of the Universe becoming transparent

Born in fire, cooled by expansion, collapsed by clumpiness.

That is the Universe in a nutshell.

A few hundred million years after the Big Bang, the first stars and galaxies condensed out of immense clouds of gas. Exactly when is a subject of intense research, but so far, astronomers have discovered galaxies all the way back to less than 300 million years after the Big Bang.

Now one of these very first galaxies turns out to reveal an interesting fact about the Universe:

Risk of dense fog with low visibility

One of the reasons that detecting the first galaxies is complicated is exactly because of the gas from which they form.

Newborn galaxies shine most brightly in energetic, ultraviolet light, or UV. But during the first some half a billion years, the gas enshrouding the galaxies, and lying between them, was neutral. Because neutral gas is very efficient at absorbing UV light, this means that only the fainter, less-energetic light can make it through the fog, making observations of the first galaxies extremely challenging.

Galaxies detected in these early epochs are simply invisible at the short UV wavelengths.

Lifting the fog

As UV radiation is emitted from the first sources of light, it slowly begins to transform the Universe. The neutral atoms that hide the galaxies are split apart by the UV light, eventually rendering the Universe transparent.

This process is known as the Epoch of Reionization, and its detailed circumstances are a subject of intense research in astronomy: When did it start, how long did it take, how did it proceed, and which sources were responsible?

Until recently, the consensus was that reionization did not begin until the Universe was around half a billion years old, completing another half billion years later.

But this notion is now challenged by a new study, led by astronomers at the Cosmic Dawn Center (DAWN) at the Niels Bohr Institute and DTU Space. Investigating one of the most distant galaxies, the researchers have discovered a clear sign of reionization beginning significantly earlier than hitherto thought.

Joris Witstok, postdoc at DAWN who led the study, explains:

“Young galaxies shine brightest at a very specific wavelength of light, originating from hydrogen. To astronomers, this light goes under the name “Lyman alpha”. Because of its short UV wavelength, it is easily absorbed by the surrounding medium, and therefore no galaxy from when the Universe was less than half a billion years old has showed us this particular kind of light.”

Galactic bubbles

That is, until now.

What Witstok and his team found was that one of the very most distant galaxies, known as JADES-GS-z13-1, is gleaming brilliantly with Lyman alpha light. 

But how can Lyman alpha escape a galaxy cloaked in dense, neutral gas?

“We know from our theories and computer simulations, as well as from observations at later epochs, that the most energetic UV light from the galaxies »fries« the surrounding neutral gas, creating bubbles of ionized, transparent gas around them,” Witstok elaborates. “These bubbles percolate the Universe, and after around a billion years, they eventually overlap, completing the epoch of reionization. We believe that we have discovered one of the first such bubbles.”

In other words, the detection of the Lyman alpha light is a telltale signature of an ionized bubble, because it would not be able to escape otherwise.

Only with Webb

The observations would not have been possible without the sensitivity of James Webb and its ability to explore the light of galaxies wavelength by wavelength.

“We knew that we would find some of the most distant galaxies when we built Webb,” says Peter Jakobsen, affiliated professor at DAWN, project scientist behind James Webb’s spectrograph NIRSpec, and second-author of the study. “But we could only dream of one day being able to probe them in such detail that we can now see directly how they affect the whole Universe.”

The question remains what exactly is the cause of the ionized bubble. Although the first stars are thought to be very hot and extremely UV bright, there is another possibility:

“Most galaxies are known to host a central, supermassive black hole. As these monsters engulf surrounding gas, the gas is heated to millions of degrees, making it shine brightly in X-rays and UV before disappearing forever. This is another viable cause of the bubbles, which we will now investigate,” Witstok concludes.