We finally know what brought light into the dark, formless void of the early Universe.
According to data from the Hubble and James Webb Space Telescopes, the origin of the free-flying photons in the early cosmic dawn were small dwarf galaxies that flared into life, clearing the cloudy hydrogen fog that filled intergalactic space. A new paper about the research was published in February.
“This discovery reveals the crucial role played by ultra-faint galaxies in the early evolution of the Universe,” said astrophysicist Iryna Chemerynska of the Institut d’Astrophysique de Paris.
“They produce ionizing photons that transform neutral hydrogen into ionized plasma during cosmic reionization. This highlights the importance of understanding low-mass galaxies in shaping the history of the Universe.”
At the beginning of the Universe, within minutes of the Big Bang, space was filled with a hot, dense fog of ionizing plasma. What little light there was would not have penetrated this fog; the photons would have simply scattered the free electrons floating around, effectively making the Universe dark.
As the Universe cooled, after about 300,000 years, protons and electrons began to fuse together to form neutral hydrogen (and some helium). Most wavelengths of light can penetrate this neutral medium, but the light sources had not prevented him from producing it. But from this hydrogen and helium, the first stars were born.
Those first stars gave off radiation that was strong enough to strip electrons from their cores and reionize the gas. However, by this point, the Universe had expanded so much that the gas had dispersed and could not prevent light from shining out. About 1 billion years after the Big Bang, the end of the period known as the cosmic dawn, the Universe was completely reionized. Ta-da! The lights were on.
But because there is so much darkness in the cosmic dawn, and because it is so dark and far away in time and space, we have had trouble seeing what is there. Scientists thought that the sources responsible for most of the scrubbing must have been powerful—massive black holes whose accretion produces blazing light, for example, and large galaxies in the throes of star formation (small stars produce a lot of UV light).
JWST was designed, in part, to peer into the cosmic dawn and try to see what lies within it. It has been very successful, revealing all sorts of surprises about this crucial time in the formation of our Universe. Surprisingly, telescope observations now suggest that dwarf galaxies are a major player in reionization.
An international team led by astrophysicist Hakim Atek of the Institut d’Astrophysique de Paris turned to JWST data on a galaxy cluster called Abell 2744, supported by data from Hubble. Abell 2744 is so dense that space-time warps around it, forming a cosmic lens; any distant light traveling toward us through that space-time is magnified. This allowed researchers to see small dwarf galaxies close to the cosmic dawn.
Next, they used JWST to obtain detailed spectra of these small galaxies. Their analysis revealed that, not only are these dwarf galaxies the most abundant type of galaxy in the early Universe, they are much brighter than expected. In fact, the team’s research shows that dwarf galaxies outnumber large galaxies by 100 to 1, and their collective output is four times greater than the ionizing radiation commonly assumed for larger galaxies.
“These space plants put out more than enough energy together to get the job done,” Atek said. “Despite their small size, these low-mass galaxies are prolific producers of energetic radiation, and their abundance during this period is so substantial that their collective influence can transform the entire state of the Universe.”
It’s the best evidence yet for the force behind reionization, but there’s more work to do. The researchers looked at a small part of the sky; they need to make sure that their sample is not just an anomalous cluster of dwarf galaxies, but a representative sample of the entire population at the cosmic dawn.
They intend to study more regions of the cosmic lens of the sky to obtain a broader sample of early galactic populations. But in this sample alone, the results are incredibly exciting. Scientists have been searching for answers to reionization for as long as we have known about it. We are on the verge of clearing the fog.
“We have now entered uncharted territory with JWST,” said astrophysicist Themiya Nanayakkara of Swinburne University of Technology in Australia.
“This work opens up more exciting questions that we must answer in our efforts to map the evolutionary history of our origins.”
The research was published in Nature.
A version of this article was originally published in March 2024.