Small Galaxies Contributed More To The Early Universe Than Once Thought

John Wise, Georgia Tech.

Through the use of supercomputer simulations, researchers have discovered that, contrary to popular belief, dwarf galaxies not only contributed a significant amount of UV radiation to the early universe, but they also formed stars; both of which helped to determine certain properties of the universe that still exist today. The study has been published in Monthly Notices of the Royal Astronomical Society.

In the very early universe, hydrogen atoms making up normal matter were ionized, meaning that their negatively charged electrons had been knocked off and thus they were positively charged. As the universe began to cool, the hydrogen ions were able to recapture their lost electrons and therefore became neutral again. Millions of years later this cool hydrogen gas gave rise to the first stars which brought the universe out of the “dark age.”

Around 200 million years after stars began to form, stellar UV radiation started to ionize hydrogen gas again and thus the universe became reionized; a process that took around 800 million to complete. While this much is known, there exists some discrepancy over the factors that contributed to the reionization. The most logical explanation is that the larger galaxies would supply more UV radiation to fuel the process, but the latest study has suggested that this may not have been the case and that dwarf galaxies chipped in more than their fair share.

By modeling the flow of UV radiation through the gas as galaxies formed, the researchers discovered that the smallest and faintest galaxies contributed around 30% of the UV light during reionization. Furthermore, they found the idea that dwarf galaxies did not form stars during this time is likely incorrect.

“It turns out these dwarf galaxies did form stars, usually in one burst, around 500 million years after the Big Bang,” said study leader John Wise in a news-release. “The galaxies were small, but so plentiful that they contributed a significant fraction of UV light in the reionization process.”

The simulations also revealed that of all the ionizing photons released by stars in dwarf galaxies, around 50% escaped into intergalactic space. In contrast, a mere 5% escaped from larger galaxies. According to Wise, this is because smaller galaxies have less interstellar gas than larger galaxies which allows photons to escape before they are absorbed. It is this elevated release of UV light married with the abundance of smaller galaxies that meant they played a pivotal role in reionization.

“That such small galaxies could contribute so much to reionization is a real surprise,” said study author Michael Norman. “Once again, the supercomputer is teaching us something new and unexpected; something that will need to be factored into future studies of reionization.”

Check out a rendering of the simulation here:

 

[Via Royal Astronomical Society

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