Webb Answers One of the Early Universe’s Biggest Questions

Webb helps scientists answer one of the most persistent questions about the nature of the early Universe.
This image shows the galaxy EGSY8p7, a bright galaxy in the early Universe. This galaxy emits light and excited hydrogen atoms, known as Lyman-α emission, among other things. This galaxy was identified within a field of young galaxies studied by Webb in its CEERS survey. Previous observations had seen only one larger galaxy, rather than a galaxy with two companion galaxies. The discovery of this cluster of interacting galaxies is critical to understanding hydrogen emissions from the early Universe.

While the James Webb Space Telescope (JWST) has delivered many beautiful images, its primary mission is to probe the early Universe and help humanity solve some of the most significant questions it faces. Webb has delivered on this promise by revealing, for the first time, what is in the local environment of galaxies in the very earliest days of the Universe.

It is essential first to understand how light travels. As a brief refresher, the speed of light is 299,792,458 meters per second, or put another way, about 300,000 kilometers per second. The further away something, like a galaxy, is from the point of observation, the longer it takes its light to arrive. The James Webb Space Telescope has detected light from 13.4 billion years ago. That means the light from an ancient galaxy, like GN-z11, traveled through the cosmos for 13.4 billion years before reaching Webb.

Observing something so distant and faint is no easy feat, which is part of what makes Webb so special. The space telescope offers unprecedented resolving power and sensitivity, enabling it to peer further — and further back — than a telescope like Hubble, which is itself no slouch.

As the European Space Agency (ESA) explains, an international team of astronomers has put Webb’s prowess to good use and addressed one of the biggest puzzles of the early Universe — why it is possible to observe hydrogen atoms from the early Universe when these should have been “entirely blocked by the pristine gas that formed after the Big-Bang.”

The ESA sets the scene:

The very earliest galaxies were sites of vigorous and active star formation, and as such were rich sources of a type of light emitted by hydrogen atoms called Lyman-α emission. However, during the epoch of reionization, an immense amount of neutral hydrogen gas surrounded these areas of active star formation (also known as stellar nurseries). Furthermore, the space between galaxies was filled by more of this neutral gas than is the case today. The gas can very effectively absorb and scatter this kind of hydrogen emission, so astronomers have long predicted that the abundant Lyman-α emission released in the very early Universe should not be observable today.

However, these very early hydrogen emissions have been observed by astronomers, so researchers have been left scratching their heads. The hydrogen should have been absorbed or scattered and thus not observable today, so what is happening?

“One of the most puzzling issues that previous observations presented was the detection of light from hydrogen atoms in the very early Universe, which should have been entirely blocked by the pristine neutral gas that was formed after the Big-Bang. Many hypotheses have previously been suggested to explain the great escape of this ‘inexplicable’ emission,” explains Callum Witten, researcher at the University of Cambridge and principal investigator on the new study.

Webb helps scientists answer one of the most persistent questions about the nature of the early Universe.
This image shows EGSY8p7 and was newly processed using NIRCam data captured with seven different near-infrared filters. “This discovery of a cluster of interacting galaxies sheds light on the mystery of why the hydrogen emission from EGSY8p7, shrouded in neutral gas formed after the Big Bang, should be visible at all. Astronomers have concluded that the intense star-forming activity within these interacting galaxies energised hydrogen emission and cleared swathes of gas from their surroundings, allowing the unexpected hydrogen emission to escape,” explains the ESA.

Using Webb’s extraordinary resolution and sensitivity and its NIRCam instrument, the researchers “were able to resolve smaller, fainter galaxies that surround the bright galaxies from which the ‘inexplicable’ hydrogen emission had been detected.” Put another way, they discovered that the cosmic neighborhood is busier than anyone realized, and there are many more small, faint galaxies than expected.

Critically, these newfound smaller galaxies not only exist, which is a notable discovery in and of itself, but that they have interacted and merged. These mergers are crucial to explaining the mysterious emissions from the earliest galaxies.

“Where Hubble was seeing only a large galaxy, Webb sees a cluster of smaller interacting galaxies, and this revelation has had a huge impact on our understanding of the unexpected hydrogen emission from some of the first galaxies,” says Stanford University-based team member, Sergio Martin-Alvarez.

Building upon real-world observations, the team then used state-of-the-art computer simulations to explore the physical processes of these faint galaxies in the early Universe.

“They found that the rapid build-up of stellar mass through galaxy mergers both drove strong hydrogen emission and facilitated the escape of that radiation via channels cleared of the abundant neutral gas. So the high merger rate of the previously unobserved smaller galaxies presented a compelling solution to the long-standing puzzle of the ‘inexplicable’ early hydrogen emission,” ESA explains.

This groundbreaking research demonstrates the importance of resolving power, as with the significantly better resolution that Webb offers, it was possible to see entire galaxies that are instrumental in explaining the nature of the early Universe.

The new findings have been published today in Nature Astronomy.

Image credits: ESA/Webb, NASA & CSA, S. Finkelstein (UT Austin), M. Bagley (UT Austin), R. Larson (UT Austin), A. Pagan (STScI), C. Witten, M. Zamani (ESA/Webb)