Mysterious radio bursts traced to the edge of an ancient, dying galaxy

In February 2024 scientists on Earth detected a powerful radio burst from space. In an effort to discover where it came from, they traced the brief flash of energy back to its alien source – and discovered something unexpected.

Scientists led by Northwestern University and McGill University have tracked a fast radio burst (FRB) to the edges of an ancient elliptical galaxy. Scientists previously thought that these rapid radio bursts, which generate more energy in one burst than our Sun generates in an entire year, were produced exclusively by young galaxies that are constantly producing new stars. However, recent investigations are described in two sister research published on January 21 in The Astrophysical Journal Letters, are causing astronomers to reconsider the potential variety of FRB sources.

Named FRB 20240209A, the February 2024 FRB wasn’t just one and done. Between February and July 2024, the same source erupted 21 times.

“The prevailing theory is that FRBs come from magnetars formed by core-collapse supernovae,” said Northwestern University’s Tarraneh Eftekhari, who participated in both studies, at a Univ. statement. Magnetars are neutron stars with very strong magnetic fields, and neutron stars are extremely small and dense celestial objects that are thought to have formed as a result of the explosive death of some large stars, that is, supernovae.

“That doesn’t seem to be the case here,” Eftehari continued. “While young, massive stars end their lives as supernovae with core collapse, we see no evidence of young stars in this galaxy. Thanks to this new discovery, a picture emerges that shows that not all FRBs come from young stars. Perhaps there is a subpopulation of FRBs that are associated with older systems.

The old galaxy in question is 11.3 billion years old and 2 billion light years away. Using computer simulations, Eftehari and her colleagues found that the galaxy is extremely bright and 100 billion times more massive than our Sun.

“This appears to be the most massive FRB host galaxy to date,” Eftehari said. “It’s among some of the most massive galaxies out there.” Not only does the unusual FRB originate from an old galaxy, it also comes from the edge of that galaxy—specifically, 130,000 light-years from its center.

“Among the FRB population, this FRB is located (farthest) from the center of its host galaxy,” said McGill’s Vishwangi Shah, who participated in both studies. “This is both surprising and exciting because FRBs are expected to originate inside galaxies, often in star-forming regions. The location of this FRB so far outside its host galaxy raises questions about how such energetic events can occur in regions where no new stars are forming.

But FRB 20240209A isn’t the first FRB detected far from active star-forming regions—it’s the second. In 2022 astronomers tracked the M81 FRB, located 12 million light-years from Earth, to a cluster of stars on the edge of the Messier 81 galaxy.

FRB 20240209A “may be the twin of event M81 (M81 FRB). It is far from its home galaxy (far from where stars are born), and the population of stars in its home galaxy is extremely old. It had its heyday and is now retiring,” said Wen-fai Fong of Northwestern University, who participated in both studies. “At the same time, this type of old environment is causing us to rethink our standard FRB progenitor models and turn to more exotic channels of formation, which is exciting.”

One of the studies suggests that, like the M81 FRB, the new FRB may also originate from a star cluster called a globular cluster.

“A globular cluster origin for this recurring FRB is the most likely scenario to explain why this FRB is located outside its host galaxy,” Shah explained. “We do not know for sure if there is a globular cluster at the FRB’s position, and have submitted a proposal to use the James Webb Space Telescope for follow-up observations of the FRB’s location.” If so, that would make this FRB only the second FRB known to be in a globular cluster. If not, we will have to consider alternative exotic scenarios for the origin of FRBs.

In other words: back to the drawing board!