The farthest fast radio burst ever pinned to a galaxy came from a dwarf galaxy that existed about 3 billion years after the Big Bang, and the James Webb Space Telescope found it after the largest ground-based telescopes saw nothing at the spot. NASA announced the result Oct. 8 alongside a paper in the journal Science.
The burst, catalogued as FRB 20240304B, lasted a few milliseconds. Its host galaxy turned out to be far smaller than astronomers expected, which pushes the debate over what produces these flashes toward young, highly magnetic stars and away from colliding neutron stars.
How Astronomers Located FRB 20240304B
Fast radio bursts are millisecond-long flashes of radio emission first discovered in 2007, and most are seen only once, NASA said in its release. The MeerTRAP team caught this one on March 4, 2024, with the MeerKAT radio telescope. The radio signal suggested the burst was extraordinarily distant, but a radio telescope alone cannot say which galaxy a flash came from. That required an optical or infrared image of the exact patch of sky.
The largest ground-based telescopes could not detect any galaxy at the burst’s position. Webb’s Near-Infrared Camera did. Its Near-Infrared Spectrograph then measured the galaxy’s redshift, the stretching of its light by cosmic expansion, at 2.148, according to the preprint authors.
That number places the burst about 3 billion years after the Big Bang, NASA said. Until now the record for a localized burst was a redshift of about 1, set by a burst called FRB 20220610A, according to the full paper text. The new burst roughly doubles that reach. The paper was first posted publicly in August 2025, so the peer-reviewed version arrived about 14 months later and about 31 months after MeerKAT recorded the flash.
What the Dwarf Galaxy Says about the Source
Most known burst hosts are large, massive, star-forming galaxies. This one is not. NASA said the host is a small dwarf galaxy about 1,000 times less massive than the team expected. The paper puts its stellar mass at roughly 10 million times the mass of the Sun and its star formation at about 0.2 solar masses a year, with metal content of 10% to 20% of the Sun’s.
The galaxy was forming stars during cosmic noon, the period of peak star formation in the universe. NASA said its star-formation history suggests most of its stars formed within about 30 million years. The paper lists a stellar formation timescale of 51.7 million years, a different figure from a different fitting method. Both point to a young population.
That youth matters for the two leading theories. One holds that bursts come from merging neutron stars, a process NASA said takes billions of years and should favor older galaxies. The other points to a magnetar, a young neutron star with an extreme magnetic field that can produce a burst through a starquake soon after a supernova.
Lead author Manisha Caleb of the University of Sydney told NASA the finding makes a merger origin “very unlikely”. The paper adds that low metal content favors massive stars and therefore magnetars, and it links the idea to bursts seen from a magnetar in the Milky Way known as SGR J1935+2154. One burst cannot settle the question, and the paper frames the young-star channel as favored by this host rather than proven for all bursts.
How the Burst Measured Gas along Its Path
A radio burst also carries a record of what it passed through. Free electrons in space slow low-frequency radio waves more than high-frequency ones, and the total delay is called the dispersion measure. The paper reports an observed dispersion measure of 2462.48 parsecs per cubic centimeter, or 2458.20 after correcting for scattering in the signal.
The sightline crosses the Virgo Cluster, which lies about 54 million light-years away, NASA said. The paper estimates Virgo alone adds about 235 units, which is about 9.6% of the corrected total. The path also passes through a galaxy cluster at redshift 0.3, about 3.5 billion light-years away, that astronomers had not known about before. NASA quoted co-author J. Xavier Prochaska of the University of California, Santa Cruz, describing bursts as a flashlight for tracing the cosmic web.
The paper reports a fluence of 2.75 janskys-milliseconds and a peak flux density of 0.49 janskys. It also found a scattering time of 5.6 milliseconds at 1 gigahertz, a measure of how much the pulse was smeared on its way.
What Comes Next for Distant Radio Bursts
NASA said the team expects MeerKAT to detect and localize several bursts a year at redshifts above 1.0. Webb would then be needed to study their host galaxies, since ground telescopes struggled with this one. If those hosts also prove to be small and young, the magnetar case strengthens. If they are a mix, bursts may have more than one origin.
NASA lists Ben Stappers of the University of Manchester and Nanayakkara among the co-authors, with Caleb as lead. It credits the image of the host galaxy to NASA, ESA, CSA, STScI and Themiya Nanayakkara of the University of Sydney, with processing by Joseph DePasquale of the Space Telescope Science Institute.
