In 2007, astronomers detected a flash of radio waves that was much shorter in duration than the blink of an eye1. Such signals, now called fast radio bursts (FRBs), are thought to have been produced billions of years ago in distant galaxies2. If so, the sources of FRBs must be spectacularly energetic and, quite possibly, unlike anything that has ever been observed in our Galaxy. Pinpointing the galaxies that host FRBs is the key to unlocking the mysterious origins of these signals. Writing in Nature, Ravi et al.3 report the discovery of the likely host galaxy of an FRB that travelled for 6 billion years before reaching Earth. The properties of this galaxy suggest that active star formation is not essential for making an FRB source.
The maxim ‘location, location, location’ applies to FRBs: knowing where these signals originate is crucial to understanding what generates them. Although astronomers have detected almost 100 FRB sources so far2, the measured positions of these sources on the sky have typically been too inaccurate to identify their host galaxies. One exception is the first FRB source observed to produce repeat bursts4. This source was localized to a region of active star formation in a puny ‘dwarf’ galaxy5. The finding supported theories that ascribe the origin of FRBs to the extremely condensed remnants of powerful stellar explosions called supernovae. For example, the repeating FRBs could originate from young and hyper-magnetized neutron stars — the collapsed remnants of massive stars6.
However, most FRB sources have not been seen to produce repeat bursts. Astronomers have therefore questioned whether these apparently one-off events have a different origin from that of the repeating FRBs2. From a practical point of view, one-off FRBs are much more challenging to study than repeaters. In the case of a repeating FRB, a patient observer can wait for further bursts and refine the measured position of the source. But for a one-off FRB, the position needs to be pinpointed by capturing the necessary high-resolution data at the same time as the burst is discovered.
Ravi and colleagues achieved this feat using an array of ten relatively small (4.5-metre-diameter) radio dishes spread across an area of roughly one square kilometre in Owens Valley, California. This distributed telescope network, known as the Deep Synoptic Array 10-antenna prototype (DSA-10), can scan a broad swathe of sky for FRBs (Fig. 1a). It can also provide enough spatial resolution to determine the position of a burst on the sky with high precision7. This precision must indeed be extremely high: unless the position is known to 1,000th of a degree, robustly associating an FRB with a specific host galaxy is impossible8. Even though Ravi et al. determined the position of their FRB to this level of precision (Fig. 1b), there is still some uncertainty as to whether or not the identified galaxy is the true host.
The authors demonstrate that this likely host galaxy is markedly different from the host5 of the well-localized source of the repeating FRB. It is 1,000 times more massive, and shows none of the prodigious star formation that is associated with the environment of the repeating-FRB source. Only a week before Ravi and colleagues’ work was published online, a similar breakthrough was reported9 using the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The authors of that paper achieved an even more precise localization of another one-off FRB, and also demonstrated that it originates from a massive galaxy that shows little signs of active star formation.
So, do these results mean that one-off FRBs and repeaters come from different galaxy types, and that they have physically different origins? Do astronomers have two puzzles on their hands? Perhaps, but with only three FRB host galaxies identified so far, many alternatives remain open. For instance, it is possible that all FRBs are generated by hyper-magnetized neutron stars, but that there are various ways in which such neutron stars can be produced10. Some might form directly through the collapse of a massive star, whereas others might be made from old neutron stars in a binary system that smash into each other as the orbital distance between them decreases. This difference could explain why some FRBs seem to originate from star-forming regions and others do not10.
Excitingly, we will soon know a lot more. The mystery of FRBs has driven many teams worldwide to tune radio telescopes towards discovering and localizing these signals, and many thousands of FRBs are thought to happen somewhere on the sky each day2. The fact that fewer than 100 FRB sources have been detected is a reflection of the small fields of view of existing radio telescopes. If a sensitive radio telescope could be built that has a continuous view of the entire sky, FRBs would look like a fireworks display. However, wide-field telescopes such as the Canadian Hydrogen Intensity Mapping Experiment11 (CHIME) are starting to change the game. It might not be long before astronomers have catalogued thousands of FRB sources and pinpointed at least dozens of them.
The precise localizations from DSA-10 and ASKAP are shedding light on the origins of FRBs, but they are also teaching us about the potential use of these signals as astronomical probes. FRBs are delayed in their arrival at Earth by the otherwise invisible material between galaxies2. By measuring the magnitude of this time delay, and comparing this measurement with the distance to the host galaxy, astronomers can map the density of ionized material in intergalactic space and thereby weigh the Universe in a unique way. The localizations of one-off FRBs suggest that FRB host galaxies will only slightly skew such measurements. Moreover, the results indicate that, with the detection and localization of thousands of FRBs, a 3D map of the material between galaxies could be made.
Nature 572, 320-321 (2019)