Ancient galaxy shines through the clouds of the early universe

One of the oldest known galaxies is shedding light on the early years of the universe.

In the aftermath of the Big Bang, the universe was full of hydrogen and helium. This cosmic debris was particularly good at absorbing short wavelengths, meaning light such as ultraviolet (UV) shouldn’t have been able to travel very far.

However, one very early galaxy was shining brightly just 330 million years after the beginning of the universe. Measurements taken by the James Webb Space Telescope show that its UV light has been able to cut through the gas clouds, surprising astronomers in the process.

Dr Kevin Hainline is the co-author of new research into this unusual galaxy, which is so far only identified by the code JADES-GS-z13-1.

“We really shouldn’t have found a galaxy like this, given our understanding of the way the universe has evolved,” Kevin says. “We could think of the early Universe as shrouded with a thick fog that would make it exceedingly difficult to find even powerful lighthouses peeking through, yet here we see the beam of light from this galaxy piercing the veil.”

The astronomers hope that the findings of their study, published in the journal Nature, will lead to a better understanding of the first galaxies in the universe.

The universe’s early years

If you look up into the night sky, you can see stars that are millions of light years away. But this hasn’t always been possible, as the visibility of the universe has changed a lot over its first few hundred million years.

In the moments immediately after the Big Bang, vast clouds of hydrogen and helium condensed out of subatomic matter, forming an opaque plasma that filled the entire universe for 380,000 years.

Eventually, the universe cooled down enough that this opaque soup of charged gases formed into atoms. While this made the universe transparent for the first time, there wasn’t a lot to see – just a cloud of gas filling the cosmos.

Once the cloud of gas collapsed into the first stars and galaxies, different types of light would have started to illuminate the universe. But UV light would have been one of the few exceptions, as hydrogen is very good at absorbing it.

Until recently, it wasn’t thought that the universe became completely transparent to starlight until the end of the epoch of reionisation 12.8 billion years ago. By this point, there would have been enough stars with the energy to ionise the clouds of hydrogen atoms and split them into protons and electrons which aren’t as good at absorbing UV.

JADES-GS-z13-1, however, is almost 13.5 billion years old but still able to shine through the cosmic fog. The challenge for astronomers is to find out why.

What’s causing the light?

For the UV light to have escaped through the cloud, it would need to have emerged from a bubble of ionised hydrogen measuring at least 650,000 light years across. This is a long enough distance that the UV would have been stretched by the expansion of the universe into visible light, which isn’t trapped by hydrogen gas.

In order to form a bubble this big, JADES-GS-z13-1 must have a very powerful energy source at its heart. Dr Joris Witstok, the study’s lead author, says that powerful early stars might fit the bill.

“The large bubble of ionised hydrogen surrounding this galaxy might have been created by a peculiar population of stars,” Joris says. “These are much more massive, hotter and more luminous than stars formed at later epochs, and possibly representative of the first generation of stars.”

If this early galaxy had enough of these massive stars, which were hundreds of times bigger than the Sun, then their energy would have been enough to start ionising the hydrogen around them.

The other possibility is that the process is being caused not by the stars themselves, but their remnants – specifically, a supermassive black hole.

While nothing can escape from a black hole once it has passed across the event horizon, objects spend a lot of time spiralling towards it. As they fall, as much as 40% of their energy is released as light, including UV. This would be more than enough to ionise the surrounding hydrogen.

Both of these theories are difficult to prove, so the researchers plan to carry out further observations of JADES-GS-z13-1 and other early galaxies to find which is more likely. Their findings could reveal more about the evolution of the universe itself.