A recently detected flash of energy appears to have emanated from the wreckage of colliding galaxies, according to an international team of astronomers led by Penn State scientists. The burst, known as GRB 230906A, was likely caused by the collision of two neutron stars hundreds of millions of years ago and is now shedding light on how the universe creates some of its heaviest elements.
The signal, first detected by the NASA Fermi satellite in September 2023, belonged to a peculiar class of short gamma-ray bursts, explosions so powerful they briefly outshine entire galaxies. These bursts occur when two neutron stars - dead remnants of massive stars - spiral together and collide, unleashing a flood of energy and forging heavy elements like gold and platinum, explained Simone Dichiara, assistant research professor of astronomy and astrophysics at Penn State and lead author on a paper about the discovery published today (March 10) in The Astrophysical Journal Letters.
Using NASA's Chandra X-ray Observatory and Hubble Space Telescope, the researchers pinpointed the burst to a faint galaxy that appears to be part of a larger group of galaxies about 8.5 billion light-years away. This group is undergoing a cosmic merger - galaxies colliding and interacting, stirring up star formation. The burst occurred in the debris field of this galactic collision, a long, thin stream of stars and gas stretching across space. When galaxies interact, their gravity tugs on each other so strongly that material like stars, dust and gas is stretched out into space, forming a tail-like structure that scientists refer to as a "tidal tail."
"This could be an indication that tidal interaction between galaxies can trigger star formation and two neutron stars that evolve from the new stars can end up merging into each other, making these big explosions and energetic emissions that we observe," Dichiara said.
He added that such explosions, also called compact binary star mergers, generate kilonova emissions: bright halos of light that are one of the main sites of heavy element production in the universe.
"This could provide a natural explanation for why we see an enhanced rate of production of heavy elements in the halo of interacting galaxies," he said.
The team said they suspect the neutron stars that collided were born during a surge of star formation triggered by the galactic merger roughly 700 million years ago. Their eventual collision not only produced the powerful gamma-ray burst detected by researchers but also scattered newly forged heavy elements into surrounding space.
"We got a rare glimpse into how destruction can be a catalyst for creation," said Jane Charlton, professor of astronomy and astrophysics at Penn State and co-author on the paper. "The gold that we have on Earth was produced in an explosive event of this nature. The heavy elements in our body, like iron for example, come from about 10,000 stars that were in our galaxy and died. It took billions of years, but that iron persisted on Earth and, as our bodies formed and evolved, they used that material."
Charlton said the team's results underscore how violent interactions between galaxies can set the stage for powerful cosmic events that could alter the composition of elements in the universe. She also stressed the importance of precision X-ray imaging. Without the Chandra X-ray Observatory, the faint host galaxy might have been overlooked entirely.
For now, the burst's exact distance remains uncertain. It could be even farther away, making it one of the most distant short gamma-ray bursts ever recorded. Future observations with next-generation telescopes may settle the question, Charlton said.
"It's very common for galaxies to have neighbors. That's not unusual at all, but having them collide is," she said. "Our own Milky Way galaxy has a neighbor, the Andromeda galaxy, and four or five billion years from now, it will merge with the Milky Way galaxy. This very thing could be happening, and tidal tails will form, kicking up heavy elements and enriching the universe."
Other co-authors are Eleonora Troja and Yu-Han Yang of the University of Rome-Tor Vergata; Brendan O'Connor of Carnegie Mellon University; Paz Beniamini, affiliated with both The Open University of Israel and George Washington University; Antonio Galván-Gámez of the Universidad Nacional Autónoma de México; and Takanori Sakamoto and Yuta Kawakubo from Aoyama Gakuin University in Japan.
This research was supported by NASA, the Smithsonian Astrophysical Observatory, the European Research Council, the U.S. National Science Foundation, the U.K. Science and Technology Facilities Council and the Royal Society.
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