Photons from Dwarf Galaxies Reionized Early Universe, New Research Suggests

The reionization of the Universe occurred some 500 to 900 million years after the Big Bang. It represents the transformation of neutral hydrogen into ionized gas and marks the end of the ‘Dark Ages’ in cosmic history. Now, astronomers using the NASA/ESA/CSA James Webb Space Telescope have obtained the spectra of eight ultra-faint dwarf galaxies that existed less than a billion years after the Big Bang. Their observations could help settle a long-running scientific debate about the drivers of reionization and could be essential to understanding the formation of the very first galaxies.

Much remains to be understood about the time in the Universe’s early history known as the era of reionization.

It was a period of darkness without any stars or galaxies, filled with a dense fog of hydrogen gas, until the first stars ionized the gas around them and light began to travel through.

Astronomers have spent decades trying to identify the sources that emitted radiation powerful enough to gradually clear away this hydrogen fog that blanketed the early Universe.

“Our discovery unveils the crucial role played by ultra-faint galaxies in the early Universe’s evolution,” said Dr. Iryna Chemerynska, an astronomer at the Institut d’Astrophysique de Paris.

“They produce ionizing photons that transform neutral hydrogen into ionized plasma during cosmic reionization.”

“It highlights the importance of understanding low-mass galaxies in shaping the Universe’s history.”

“These cosmic powerhouses collectively emit more than enough energy to get the job done,” said Dr. Hakim Atek, also from the Institut d’Astrophysique de Paris.

“Despite their tiny 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.”

In their study, the astronomers captured and analyzed the spectra of eight extremely faint galaxies magnified by the lensing cluster Abell 2744.

They found that these galaxies are immense producers of ultraviolet light, at levels that are four times larger than what was previously assumed.

This means that most of the photons that reionized the Universe likely came from these dwarf galaxies.

“We have now entered uncharted territory with Webb,” said Dr. Themiya Nanayakkara, an astronomer at the Swinburne University of Technology.

“Our work opens up more exciting questions that we need to answer in our efforts to chart the evolutionary history of our beginnings.”

The results were published in the journal Nature.

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H. Atek et al. 2024. Most of the photons that reionized the Universe came from dwarf galaxies. Nature 626, 975-978; doi: 10.1038/s41586-024-07043-6