Webb Places the Farthest Fast Radio Burst Inside a Tiny Young Galaxy

On March 4, 2024, a radio array in South Africa caught a flash that lasted only a few milliseconds and then vanished. MeerKAT, working with the MeerTRAP search system, logged it at 00:50 UTC and gave it the name FRB 20240304B. The pulse arrived stretched and delayed by free electrons along the path, with a dispersion measure near 2,458, a peak of about half a jansky, and a scattering time of 5.6 milliseconds at 1 gigahertz. Those numbers already hinted that the source sat far beyond nearly every localized burst on record.

Ground-based telescopes were unable to detect the signal-sending galaxy. A one-hour exposure with the Keck Observatory in Hawaii yielded no results, indicating that the host galaxy is too faint for any sensor on Earth to detect. The Webb telescope’s near-infrared camera detected a faint smudge in the exact position in the sky, the Virgo constellation, and the near-infrared spectrograph identified strong hydrogen and oxygen lines. However, the redshift was shockingly high, measuring 2.148. NASA estimates that the galaxy is around 10.63 billion light-years away, implying that the burst occurred when the universe was only 3 billion years old, at what astronomers refer to as cosmic noon, when star creation was at its peak.

What the team saw on the image was a tiny dwarf galaxy, not the large, bustling star factory that most other bursts originate from. This little star has a mass of roughly 8 million suns, which is a minuscule fraction of what the researchers had expected, and it is metal poor, containing less than 10 to 20% of the metal in the Sun. The galaxy produces new stars at a rate of around 0.2 solar masses per year, and the team believes that all of the stars developed in about 30 million years. Manisha Caleb of the University of Sydney, who led the study published in Science, said they expected a large, mature galaxy but instead discovered a small, dwarf galaxy that was still producing stars at a rapid pace.

It now appears more unlikely that the flare resulted from the merging of two neutron stars, given such collisions take billions of years and only occur in older star populations. On the other hand, a young neutron star left over from a supernova may be able to explain the timing of the flash. In that case, a sudden shift in the neutron star’s crust, known as a starquake, can unleash a massive amount of energy into a radio pulse very immediately after the star dies. Caleb’s team believes the merger route is unlikely for FRB 20240304B, due to the dwarf galaxy’s age, which is a major concern.

Plus, the same pulse detected a couple of interesting marks from normal matter that would be difficult to map otherwise. One of these marks comes from the Virgo Cluster, which is approximately 54 million light-years away from Earth. The other is from a galaxy group that appears to have never been cataloged, with a redshift of roughly 0.3 and a distance of approximately 3.5 billion light-years. According to J. Xavier Prochaska of the University of California, Santa Cruz (who helped discover that the host galaxy was just not visible from the ground), a burst like this is similar to a spotlight in that it records every structure it passes through. The detection effectively doubles what we know about the redshift reach of these concentrated fast radio bursts and provides a glimpse of what it looks like when hydrogen is ionized at 80% of the universe’s total age. MeerKAT is predicted to detect a large number of these events at redshift 1 each year, and the Webb telescope is presently the only tool capable of identifying the galaxies from which they originate.