Researchers have discovered that fast radio bursts (FRBs) are more frequently found in large, star-forming galaxies rather than in their smaller counterparts.
Since they were first identified in 2007, fast radio bursts—intense bursts of radio light—have intrigued astronomers, prompting efforts to uncover their origins. Currently, there are hundreds of confirmed FRBs, and scientists believe they are triggered by highly magnetic neutron stars called magnetars (a type of deceased star). A key finding emerged when a magnetar in our own galaxy exploded, and observatories such as Caltech’s STARE2 (Survey for Transient Astronomical Radio Emission 2) captured the event live.
In a new study published in the journal Nature, researchers from Caltech have identified that FRBs predominantly occur in massive star-forming galaxies rather than in low-mass star-forming ones. This insight has prompted fresh thoughts on the formation of magnetars themselves, indicating that these remarkable neutron stars—which have magnetic fields 100 trillion times stronger than that of Earth—often arise from the merging of two stars, which subsequently explode as a supernova. Previously, it was uncertain if magnetars were formed through such a merger or solely from the explosion of a single star.
“The incredible energy of magnetars makes them some of the most intriguing and extreme objects in the cosmos,” says Kritti Sharma, the lead author of the study and a graduate student under Vikram Ravi, a Caltech assistant professor of astronomy. “There’s limited knowledge about what triggers the formation of magnetars from the death of massive stars. Our research contributes to answering this question.”
The project initiated with an investigation for FRBs using the Deep Synoptic Array-110 (DSA-110), a Caltech project supported by the National Science Foundation and situated at the Owens Valley Radio Observatory near Bishop, California. So far, this extensive radio array has detected and pinpointed 70 FRBs to their exact galaxies—significantly more than the 23 FRBs localized by other telescopes. The researchers focused on analyzing 30 of these identified FRBs.
“DSA-110 has increased the known number of FRBs with identified host galaxies by more than double,” comments Ravi. “This was our goal for the array.”
Although FRBs are acknowledged to emerge from galaxies that are actively generating stars, the team was surprised to find that these bursts are more prevalent in massive star-forming galaxies compared to smaller ones. This finding challenges the previous belief that FRBs occurred across all types of active galaxies.
With this new understanding, the researchers began to consider the implications for FRBs. Massive galaxies tend to have higher metal content because the heavier elements in our universe, created by stars, accumulate over cosmic time. The observation that FRBs are more frequent in these metal-rich galaxies suggests that magnetars, the source of FRBs, are also more common in these environments.
Stars with higher metal content—defined in astronomy as elements heavier than hydrogen and helium—generally grow larger than other stars. “As galaxies evolve, each successive generation of stars enriches the galaxies with metals as they age and die,” states Ravi.
Moreover, massive stars that can explode as supernovae and become magnetars are usually found in pairs; about 84 percent of massive stars are in binary systems. When one massive star in a binary pair expands due to extra metal, its excess material is transferred to its companion star, promoting their eventual merger. The merged stars would collectively possess a stronger magnetic field compared to a single star.
“When a star has a higher metal content, it expands and transfers mass to its partner, leading to a merger, resulting in an even more massive star with a cumulative magnetic field greater than that of an individual star,” explains Sharma.
In summary, since FRBs are primarily observed in large and metal-rich star-forming galaxies, it is likely that magnetars (which are believed to generate FRBs) also originate in these metal-rich settings that are conducive to the mergers of stars. This suggests that magnetars throughout the cosmos arise from the remnants of stellar mergers.
Looking ahead, the team aims to continue locating more FRBs and their origins using DSA-110, and eventually using the upcoming DSA-2000, a larger radio array set to be constructed in the Nevada desert and expected to be completed by 2028.
“This achievement marks a significant milestone for the entire DSA team. Many of the authors of this paper contributed to the development of DSA-110,” says Ravi. “The effectiveness of DSA-110 in localizing FRBs indicates promising prospects for the future success of DSA-2000.”
