Webb has collected new data that sheds light on the end of the “dark ages” of the Universe
For about 400,000 years after the Big Bang, space was a very dark place. The glow caused by the explosive birth of the universe cooled, and space was filled with dense gas—mostly hydrogen—without any light sources.
Slowly, over hundreds of millions of years, the gas was pulled together by gravity into clumps that eventually became large enough to trigger nuclear fusion. These were the first stars.
At first, their light did not travel far, as most of it was absorbed by the hydrogen gas mist. However, as more and more stars formed, they produced enough light to burn away the fog, “reionizing” the gas, resulting in the transparent Universe dotted with bright points of light that we see today.
But which stars exactly produced the light that ended the Dark Ages and triggered the so-called “era of reionization”? IN researchpublished in the journal Nature, astronomers used a giant galaxy cluster as a magnifying glass to peer at faint relics of the time—and found that stars in small, dim dwarf galaxies were most likely responsible for this cosmic-scale transformation.
What brought an end to the Dark Ages?
Most astronomers already agreed that galaxies were a major force in the reionization of the Universe, but it was unclear how they did it. We know that stars in galaxies must produce many ionizing photons, but these photons must escape the dust and gas within their own galaxy to ionize hydrogen in the space between galaxies.
It is not yet clear which galaxies are capable of producing and emitting enough photons to cope with this task. (And indeed, there are those who believe that more exotic objects such as large black holes may be responsible).
Among galaxy theory adherents there are two camps.
The first believes that ionizing photons created huge, massive galaxies. There weren't many such galaxies in the early Universe, but each one produced a lot of light. So if some of this light managed to escape, this could be enough to reionize the Universe.
The second camp believes that we are better off ignoring giant galaxies and focusing on the vast number of much smaller galaxies in the early Universe. Each of them would produce much less ionizing light, but due to their large numbers they could stimulate the era of reionization.
Magnifying glass 4 million light years wide
Trying to look at anything in the early Universe is very difficult. Massive galaxies are rare, making them difficult to find. Smaller galaxies are more common, but they are very faint, making it difficult (and expensive) to obtain high-quality data.
We wanted to take a look at some of the faintest galaxies, so we used a huge group of galaxies called the Pandora Cluster as a magnifying glass. The cluster's enormous mass distorts space and time, amplifying light from objects behind it.
As part of UNCOVER, we used the James Webb Space Telescope to look at magnified infrared images of faint galaxies beyond the Pandora Cluster.
We first looked at a variety of different galaxies, and then selected a few particularly distant (and therefore ancient) ones for closer study. (Such detailed study is expensive, so we were only able to look at eight galaxies in more detail.)
The bright glow of hydrogen
We selected several sources whose brightness was about 0.5% of the brightness of our Milky Way galaxy at the time and tested them for the characteristic glow of ionized hydrogen. These galaxies are so faint that they were only visible due to the magnifying effect of the Pandora Cluster.
Our observations confirmed that these small galaxies did indeed exist in the very early Universe. What's more, we've confirmed that they emit about four times more ionizing light than we consider “normal.” This is the highest limit of what we predicted based on our understanding of how early stars formed.
Because these galaxies produced so much ionizing light, only a small portion of it had to escape to reionize the Universe.
We previously believed that for ionizing photons to become the dominant factor in reionization, about 20% of all ionizing photons would have to escape from these small galaxies. Our new data suggests that even 5% is enough – this is approximately the fraction of ionizing photons that we observe emerging from modern galaxies.
So now we can say with confidence that these small galaxies could have played a very large role in the era of reionization. However, our study was based on only eight galaxies located near the same line of sight. To confirm our results, we will need to look at other parts of the sky.
We are planning new observations that will focus on other large galaxy clusters in other parts of the Universe to find even more magnified and faint galaxies to test. If everything goes well, then in a few years we will get answers to some questions.
