Nearby galaxy might explain what tore apart universe’s hydrogen

Radiation from star cluster hints at what caused epoch of reionization after the Big Bang

galaxy in UV light

RADIANT GLOW  Ultraviolet light (orange) pours out of a galaxy in this image from NASA’s GALEX satellite. New Hubble observations reveal this galaxy might mimic the earliest galaxies in the universe.

NASA; ESA; R. Overzier/ON/MCT; T. Heckman/JHU

A nearby galaxy is leaking clues about one of the biggest makeovers in the history of the universe. New observations show that tiny galaxies in the early universe could have triggered the epoch of reionization — a period when harsh radiation tore apart hydrogen atoms — which astronomers consider key to understanding how stars and galaxies arose from the universe’s early dark void.

“Reionization is one of the major milestones in the universe’s history,” says Brant Robertson, an astronomer at the University of Arizona in Tucson. In the cosmic dark ages, neutral hydrogen gas filled the universe. Then, sometime within a billion years after the Big Bang, ultraviolet radiation filled the universe and ripped electrons from all the hydrogen atoms, leaving them ionized. Astronomers suspect that the radiation came from intense bursts of star formation in the first generations of galaxies.

But researchers weren’t sure if the tiny galaxies that populated the early universe could produce enough radiation to ionize all of space. Similar galaxies that are close to the Milky Way don’t release much ionizing radiation; the ultraviolet light from newborn stars isn’t intense enough to puncture the thick hydrogen clouds that swaddle stellar nurseries.

However, Sanchayeeta Borthakur, an astronomer at Johns Hopkins University, and colleagues suspected that J0921+4509, a compact galaxy about three billion light-years away in the constellation Ursa Major, might be leaking ultraviolet light. In the galaxy’s center, more than a billion stars are crowded into a community nearly 700 light-years across, from which stellar winds gust at almost 4 million kilometers per hour. The mass and size of the galaxy are nearly identical to the most distant, earliest galaxies known.

When Borthakur’s team pointed the Hubble Space Telescope toward J0921+4509, the researchers found that the galaxy floods the surrounding space with ionizing light through holes punched in the enveloping hydrogen clouds, the researchers report in the Oct. 10 Science.

J0921+4509 demonstrates that if you put a lot of stars in a very compact region, then they can be powerful enough to carve tunnels in the surrounding gas and leak radiation, Borthakur says: “This shows how galaxies which are really tiny can influence the entire universe.”

Ionizing light from the earliest galaxies remains hidden, Robertson says, because any ultraviolet photons that do manage to escape will be absorbed by a neutral hydrogen atom somewhere along the long journey to Earth. But by finding nearby leaky galaxies, he says, astronomers can count how many of those photons manage to escape. Researchers can then use those results to infer what might have happened 13 billion years ago.

The next step is to figure out if there were enough compact galaxies a few hundred million years after the Big Bang to ionize the universe. The new results indicate that the team’s earlier suspicion that this galaxy is leaky, which was based on indirect means, was correct, Borthakur notes. Since those earlier measurements are easier to make, the researchers plan to apply them to many tiny nearby galaxies. 

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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