Maunakea, Hawaiʻi — For the first time, astronomers have traced a fast radio burst (FRB) to the outskirts of an ancient, dead, elliptical galaxy — an unprecedented home for a phenomenon previously associated with much younger galaxies.
Detailed in two complementary studies led by Northwestern University and McGill University, the discovery shatters assumptions that FRBs solely emanate from regions of active star formation. The new observational evidence, instead, hints that the origins of these mysterious cosmic events might be more diverse than previously thought.
Both studies, which include data from W. M. Keck Observatory on Maunakea, Hawaiʻi, are published today in the Astrophysical Journal Letters.
Out of the nearly 100 FRBs that have been pinpointed to a galaxy so far, most have likely originated from magnetars, which are formed through core-collapse supernovae, the explosive death of stars more massive than the Sun.
“That doesn’t appear to be the case here,” said Tarraneh Eftekhari, NASA Hubble Einstein Fellow and Radio Astronomer at Northwestern University who led one of the studies and coauthored the other. “While young, massive stars end their lives as core-collapse supernovae, we don’t see any evidence of young stars in this galaxy. Thanks to this new discovery, a picture is emerging that shows not all FRBs come from young stars. Maybe there is a subpopulation of FRBs that are associated with older systems.”
Astronomers first detected the new FRB, dubbed FRB 20240209A, in February 2024 using the Canadian Hydrogen Intensity Mapping Experiment (CHIME). While FRBs are brief, powerful radio blasts that generate more energy in one quick burst than our Sun emits in an entire year, this event flared up more than once. Between the initial burst in February through July 2024, the same source produced another 21 pulses.
“This new FRB shows us that just when you think you understand an astrophysical phenomenon, the universe turns around and surprises us,” said Wen-fai Fong, Assistant Professor at Northwestern University and a senior author on both studies. “This ‘dialogue’ with the universe is what makes our field of time-domain astronomy so incredibly thrilling.”
After the team pinpointed the FRB’s position, Eftekhari and her collaborators moved quickly to utilize the Keck Observatory and its Low Resolution Imaging Spectrometer (LRIS) to explore the event’s surrounding environment, along with the Gemini North telescope, both operating from Maunakea.
“For nearby galaxies, there is often archival data from surveys available that tells you the redshift — or distance —to the galaxy. However, in some cases, these redshift measurements may lack precision, and that’s where Keck Observatory and the LRIS instrument becomes crucial,” says Yuxin (Vic) Dong, fourth-year PhD student, NSF Graduate Research Fellow and second author on one of the studies. “Using a Keck/LRIS spectrum, we can extract the redshift to a very high accuracy. Spectra are like fingerprints of galaxies, and they contain special features, called spectral lines, that encode tons of information about what’s going on in the galaxy like the stellar population age and star formation activity. What’s really fascinating in this case is that the features we saw from the Keck/LRIS spectrum revealed that this galaxy is quiescent, meaning star formation has shut down in the galaxy,” Dong said. “This is strikingly different from most FRB galaxies we know which are still actively making new stars.”
Instead of finding a young galaxy, these observations surprisingly revealed that the FRB originated at the edge of an 11.3-billion-year-old neighboring galaxy, located just 2 billion light-years from Earth.
To learn more about this unusual host galaxy, the team used high-performance computers to run simulations. They found that the galaxy is extremely luminous and incredibly massive — 100 billion times the mass of our sun – appearing to be the most massive FRB host galaxy to date.
But, while most FRBs originate well within their galaxies, the team traced FRB 20240209A to the outskirts of its home — 130,000 lightyears from the galaxy’s center where few other stars exist.
“Among the FRB population, this FRB is located the furthest from the center of its host galaxy,” said Vishwangi Shah, a graduate student at McGill, who led the effort to pinpoint the FRB’s origins. “This is both surprising and exciting, as 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 as to how such energetic events can occur in regions where no new stars are forming.”
Before this discovery, astronomers had traced only one other FRB to the outer fringes of a galaxy. In 2022, an international team of astronomers detected an FRB, which emanated from a tight cluster of stars on the edge of Messier 81 (M81), a grand design spiral galaxy located about 12 million light years from Earth. Although FRB 20240209A occurred in an elliptical galaxy, the two events share several other similarities.
“In fact, this CHIME FRB could be a twin of the M81 event. It is far from its home galaxy (far away from where any stars are being born), and the population of stars in its home galaxy is extremely old.” Fong said “This type of old environment is making us rethink our standard FRB progenitor models and turning to more exotic formation channels, which is exciting.”
The McGill-led study discusses the likelihood that the new FRB originated within a dense globular cluster. Such clusters are promising sites for magnetars possibly formed through other mechanisms and associated with older stars, including through the merger of two neutron stars or from a white dwarf collapsing under its own gravity. The researchers have submitted a proposal to use the James Webb Space Telescope for follow-up observations of the FRB location to determine its origins.
The studies, “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,” were supported by Gordon & Betty Moore Foundation, NASA, the Space Telescope Science Institute, the National Science Foundation, the David and Lucile Packard Foundation, the Alfred P. Sloan Foundation, the Research Corporation for Science Advancement, the Canadian Institute for Advanced Research, The Canadian Natural Sciences and Engineering Council of Canada, the Canada Foundation for Innovation and the Trottier Space Institute at McGill.
ABOUT LRIS
The Low Resolution Imaging Spectrometer (LRIS) is a very versatile and ultra-sensitive visible-wavelength imager and spectrograph built at the California Institute of Technology by a team led by Prof. Bev Oke and Prof. Judy Cohen and commissioned in 1993. Since then it has seen two major upgrades to further enhance its capabilities: the addition of a second, blue arm optimized for shorter wavelengths of light and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in 2011 for research determining that the universe was speeding up in its expansion.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.