A trio of bubble-blowing galaxies may offer clues about one of the greatest cosmic makeovers in the history of the universe.
Sometime during the universe’s first billion or so years, most of the hydrogen atoms in the cosmos became ionized when their electrons were torn away. Astronomers suspect that this reionization — so called because all hydrogen had been previously ionized for the first few hundred thousand years — was triggered by harsh ultraviolet light from the first generations of stars.
Now, researchers say they’ve caught a few galaxies blasting out ionizing light and stripping electrons from surrounding hydrogen just 680 million years after the Big Bang. If so, this would be the first direct evidence of a group of galaxies working together to ionize the early cosmos.
To look for ionizing galaxies, the team sought out galaxies in the remote universe emitting a specific wavelength of ultraviolet light. Neutral hydrogen absorbs this wavelength, preventing it from reaching Earth, but ionized hydrogen lets it slip by. Using the Mayall 4-meter Telescope on Kitt Peak in Arizona, Rhoads and colleagues went hunting for this light in a well-studied strip of the northern sky. They found three galaxies, huddled together, shining with the light — light that took over 13 billion years to reach Earth.
During that long-ago epoch, much of the universe’s hydrogen was still neutral. But the team argues that these three galaxies have created overlapping bubbles of ionized hydrogen in a sea of neutral hydrogen, allowing the ultraviolet light to escape the galaxies unimpeded. The largest of these bubbles is calculated to be over 6 million light-years across, an estimate based on how much ionizing light the brightest galaxy likely pumped out over its lifetime. That’s large enough for the ongoing expansion of the universe to stretch the light out to a longer wavelength during its travel time, so that by the time it reaches the edge of the bubble, it can pass through the enveloping neutral hydrogen.
While the brightest of these three galaxies was known to emit ionizing light, no one had yet noticed that its neighbors did as well, says Brant Roberston, an astrophysicist at the University of California, Santa Cruz, who was not involved with this research.
“What’s interesting about the galaxies being together is they can work together as a team,” Roberston says. “Once the bubbles around them overlap, then it becomes easier for them to start ionizing a larger region around them than if they each had to work on their own in separate little bubbles.”
These galaxies are so far away from Earth that it’s tough to measure more than a few properties about them, Rhoads says. So it’s hard to say exactly what lets the galaxies send out so much ionizing radiation.
To grapple with that question, Rhoads and others are looking closer to home. “We’re studying nearby galaxies that are similar in nature to these ones,” he says. By investigating those closer star systems, “we are able to look for trends in what galaxy properties allow ionizing photons to escape.”
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