Webb Pins Most Distant Localized Fast Radio Burst to Tiny Star‑Forming Dwarf Galaxy

Astronomers using the James Webb Space Telescope have identified the host galaxy and measured the distance of the most distant localized fast radio burst yet, tying the powerful flash to a tiny, active dwarf galaxy and strengthening the case that at least some of these mysterious events come from young magnetars.

The burst, called FRB 20240304B, was detected by the MeerKAT radio telescope’s MeerTRAP project on March 4, 2024, at 00:50:12.567 UTC. Fast radio bursts, or FRBs, are millisecond-long flashes of radio emission from deep space whose origins are still debated, especially when they are seen only once. In this case, Webb spectroscopy measured the host galaxy’s redshift at 2.148, plus or minus 0.001, meaning the signal came from a time when the universe was about 3 billion years old.

That host turned out to be far smaller and fainter than researchers expected. Webb’s NIRCam instrument found a faint galaxy about 0.3 arcseconds from the FRB position, and the team calculated a 97.5% probability that it is the true host. Rather than a large, mature galaxy, the source appears to be a low-mass, clumpy, star-forming dwarf galaxy with an estimated stellar mass of about 10 million suns, a star-formation rate of about 0.2 solar masses per year and metallicity roughly 10% to 20% of the sun’s. “We thought it would be a big, nicely formed galaxy with lots of stars, and instead it was a little dwarf galaxy, although it was actively forming stars,” said Manisha Caleb, the study’s lead author, in NASA’s Oct. 8 summary of the work.

The result matters because that kind of environment fits models in which at least some FRBs are produced by young magnetars — intensely magnetized neutron stars thought to form in the deaths of massive stars. It does not settle the origin of all FRBs, but it adds support for that explanation in this case. The finding also pushes localized FRBs into the era known as cosmic noon, when star formation across the universe was near its peak. The previous widely highlighted distance record for a localized FRB was FRB 20220610A, at a redshift of about 1.016, reported in 2023. The new measurement roughly doubles that redshift.

Webb was crucial because the burst’s radio signal alone could not securely establish its distance. FRB 20240304B had an extremely large dispersion measure — about 2458.20 parsecs per cubic centimeter, a reading that reflects how much material the signal passed through and hinted that it was very far away. But gas in the Milky Way, the host galaxy and intergalactic space can all contribute to that value, and this sightline also passes through substantial foreground material, including a galaxy group and the Virgo Cluster. Ground-based follow-up with the Keck Observatory and the MMT Observatory failed to identify any host galaxy, even in deep observations. Only Webb was able to detect the faint source and pin down its distance with spectroscopy.

“The host sticks out in the whole galaxy sample that we have. And it definitely was not what we were expecting,” Ben Stappers of the University of Manchester, a co-author, said in the NASA summary.

According to NASA’s summary and an arXiv preprint posted Aug. 3, 2025, by Caleb and colleagues, high-redshift FRBs with securely identified host galaxies could become useful tools for tracing ionized matter between galaxies across much of cosmic history.