Astronomers once believed that all fast radio bursts (FRBs) came from magnetars formed by the explosions of young, massive stars. However, the discovery of a new FRB raises doubts about this theory as it has been traced to the outskirts of an ancient galaxy that is 11.3 billion years old and devoid of young stars. “Just when you feel you have a grasp on an astrophysical phenomenon, the universe surprises us,” remarked a researcher involved in the study.
For the first time, scientists have tracked a fast radio burst (FRB) to the edges of a very old, inactive elliptical galaxy—an unusual setting for a phenomenon that was thought to be linked with younger galaxies.
This groundbreaking finding, discussed in two related studies led by researchers from Northwestern University and McGill University, challenges the notion that FRBs can only be found in areas with active star formation. The latest observational data suggests that the origins of these enigmatic cosmic events may be more varied than previously believed.
Both research papers will be published on Tuesday (Jan. 21) in the Astrophysical Journal Letters.
“The common belief is that FRBs originate from magnetars resulting from core-collapse supernovae,” explained Tarraneh Eftekhari from Northwestern, who co-led one of the studies. “However, in this case, we see no signs of young stars, which contradicts that idea. This discovery indicates that not all FRBs are linked to young stars, suggesting the possibility of a subset associated with older systems.”
“This new FRB illustrates that the universe continues to challenge our understanding of astrophysical events,” added Wen-fai Fong from Northwestern, a senior author on both studies. “This ongoing ‘conversation’ with the universe is what makes our field of time-domain astronomy so exhilarating.”
Eftekhari holds a NASA Einstein Fellowship at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), while Fong is an associate professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and also affiliated with CIERA.
A first for the CHIME outrigger telescopes
Astronomers first detected the new FRB, labeled FRB 20240209A, in February 2024 using the Canadian Hydrogen Intensity Mapping Experiment (CHIME). These quick, intense radio bursts release more energy in a matter of milliseconds than our sun does in an entire year.
Remarkably, this particular FRB emitted additional pulses; from the original burst in February to July 2024, the same source produced 21 more pulses—six of which were also captured by an outrigger telescope 60 kilometers away from the main CHIME station. These smaller telescopes allow astronomers to accurately determine the specific locations of FRBs.
The most massive FRB host galaxy identified
Once the team accurately located the FRB, Eftekhari and her colleagues quickly employed telescopes at the W.M. Keck and Gemini observatories to investigate the surrounding environment. Using remote access, Northwestern astronomers were able to swiftly respond to this intriguing find.
To their surprise, instead of identifying a young galaxy, the observations pointed to the FRB originating from the outskirts of an 11.3-billion-year-old galaxy, situated just 2 billion light-years from Earth.
To better understand this unusual host galaxy, the researchers utilized high-performance computers for simulations. They discovered that this galaxy is exceptionally luminous and incredibly massive—about 100 billion times the mass of our sun.
“It appears to be the most massive host galaxy for an FRB identified so far,” said Eftekhari. “It’s among the heaviest galaxies out there.”
A far-flung home
While most FRBs are found within their host galaxies, FRB 20240209A was traced to a location that is 130,000 light-years from the center of its galaxy, where very few other stars exist.
“This FRB is situated the furthest from its galaxy’s center compared to other FRBs,” noted Vishwangi Shah, a graduate student at McGill who spearheaded the efforts to trace the FRB’s origins. “This is both surprising and intriguing, as FRBs are typically found inside galaxies, often in areas where new stars are forming. The position of this FRB so far from its host galaxy raises questions about how such intense events can happen in regions where star formation has ceased.”
‘Twinning’ FRBs
Prior to this finding, only one other FRB had been located at the edge of a galaxy. In 2022, astronomers found an FRB from a tight cluster of stars on the periphery of Messier 81 (M81), a grand spiral galaxy about 12 million light-years away. While FRB 20240209A originated in an elliptical galaxy, both events possess several similarities.
“Years ago, the discovery of the M81 FRB within a dense star cluster, known as a globular cluster, shifted our thinking and led us to investigate other progenitor scenarios for FRBs,” explained Fong. “Since then, we believed that was a unique occurrence—until now.
“In fact, this latest CHIME FRB might be similar to the M81 case. It is also distanced from its home galaxy (and the area where stars are forming), and its host galaxy contains an old population of stars past their prime. This old environment prompts us to reconsider our conventional models for FRB origins and explore more intriguing formation possibilities.”
Of the nearly 100 FRBs that have been assigned to galaxies so far, most likely came from magnetars formed through core-collapse supernovae. However, the origins of FRB 20240209A could be akin to the one discovered in M81.
Possible explanations
The study led by McGill discusses the potential that the new FRB might have emerged from a dense globular cluster. These clusters are prospective sites for magnetars, possibly formed through other processes associated with older stars, such as the merger of two neutron stars or the collapse of a white dwarf under its own gravity.
“A globular cluster origin for this repeating FRB seems to be the most plausible explanation for its location outside its host galaxy,” Shah stated. “We are currently uncertain if a globular cluster exists at the FRB’s location and have proposed to use the James Webb Space Telescope for follow-up observations. If one is found, it would be only the second FRB known to originate from a globular cluster. If not, we’ll need to entertain other unconventional possibilities regarding the FRB’s origin.”
“It’s evident there are still many fascinating discoveries to be made concerning FRBs,” Eftekhari concluded, “and their surroundings may hold crucial insights into unraveling their mysteries.”
The studies, titled “A repeating fast radio burst source in the outskirts of a quiescent galaxy” and “The massive and quiescent elliptical host galaxy of the repeating fast radio burst FRB 20240209A,” received support from various organizations, including the Gordon & Betty Moore Foundation, NASA, and the National Science Foundation.
