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What a tiny green galaxy reveals about the mysteries of the cosmos

A long-held hypothesis is that newly-formed dwarf galaxies were the key to heat about 13 billion years ago. But astronomers have struggled to confirm it for twenty years.

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    This is a Hubble Space Telescope image of the compact green pea galaxy J0925. The diameter of the galaxy is approximately 6,000 lightyears, and about twenty times smaller than the Milky Way.
    Courtesy of Ivana Orlitová/Astronomical Institute/Czech Academy of Sciences
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Scientists are still debating how the universe formed some 14 billion years ago.

For the last two decades, one question in particular has stymied researchers: How did the universe heat up and reionize the all-important element hydrogen after the Big Bang?

A long-held hypothesis is that newly-formed dwarf galaxies were the key to heat about 13 billion years ago – a theory that newly-published research in Nature largely affirms. The breakthrough may help future astronomers clarify the universe’s mysterious formative years.

“The finding is significant because it gives us a good place to look for probing the reionization phenomenon, which took place early in the formation of the universe that became the universe we have today,” said lead author Trinh Thuan, an astronomer from the University of Virginia, in a statement.

Professor Thuan and his colleagues answered the long-held question by examining a type of galaxy discovered only nine years ago: “green pea” galaxies. Such galaxies are a rare kind in the nearby universe. They appear green to UV light sensors, and are round and relatively small – like a pea. Important for the sake of this research, they were also believed to experience powerful stellar explosions and winds strong enough to “eject” ionizing photons.

The team looked at a database of more than a million galaxies in the Sloan Digital Sky Survey and singled out some 5,000 galaxies for further inspection to see if any were ejecting powerful UV radiation. From 5,000 galaxies, five were chosen for observation with the Hubble Space Telescope.

With Hubble’s ability to pick up UV radiation, the team found their “green pea” galaxy in J0925+1403, located three billion light-years from Earth. The dwarf was giving off ionizing photons into the space between galaxies with a power never before observed. The researchers believe it is action like this that reionized hydrogen – the universe's most abundant element – 13 billion years ago.

After the Big Bang, the universe was so hot and dense that matter was ionized instead of neutral. But over 380,000 years, as the universe expanded outward, matter cooled down enough to become neutral. From there, the universe’s first structures were created in the form of hydrogen and helium gas clouds.

Gravity pulled these gas clouds causing them to expand in mass, eventually collapsing to make the first stars and galaxies. Then about one billion years after the Big Bang, another step: the universe reheated, and hydrogen became ionized for a second time, just like it had shortly after the Big Bang.

“As we make additional observations using Hubble, we expect to gain a much better understanding of the way photons are ejected from this type of galaxy, and the specific galaxy types driving cosmic reionization,” Trinh said. “These are crucial observations in the process of stepping back in time to the early universe.”

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