Homing In on Host Galaxies of Fast Radio Bursts

Where do fast radio bursts come from? New research shows that the nearby host galaxies of these fleeting flashes have something in common, which may help researchers understand the origins of fast radio bursts.

Mysterious Bursts

Fast radio bursts are powerful, milliseconds-long flashes of radio waves of unknown origin. Since the first fast radio burst was discovered in 2007, astronomers have detected roughly a thousand of these mysterious signals. The source of these bursts is still up for debate, with supernovae, magnetars, colliding objects, and other energetic phenomena tapped as candidates.

To understand the origin of these bursts, it helps to know what kind of galaxies they happen in. If fast radio bursts emerge from spiral galaxies with active star formation, it could mean that the bursts are linked to “prompt” formation channels such as the deaths of short-lived massive stars. If instead bursts come from elliptical galaxies with little or no star formation, that could imply that bursts come from “delayed” channels like the slowly progressing mergers of stellar remnants. So, what kind of galaxies do fast radio bursts tend to come from?

In the Neighborhood

Mohit Bhardwaj (Carnegie Mellon University and McGill University) and collaborators turned to data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) to answer this question. Their aim was to study fast radio bursts in the local universe since a sample of nearby bursts is less likely to be affected by observational biases. To find nearby bursts, Bhardwaj’s team searched the first CHIME fast radio burst catalog for signals with low dispersion measure. On average, the farther the source, the larger the dispersion measure — essentially, because there’s more stuff in between the source and Earth to disperse the radio signal.

The team found four cataloged bursts with dispersion measure excess (the amount left over after the contribution from the Milky Way is subtracted off) less than 100 parsecs per cubic centimeter, which corresponds to a distance of about 1.3 billion light-years. After searching the localization regions of these bursts in deep optical images from the Panoramic Survey Telescope and Rapid Response System survey, the authors found only one plausible host galaxy for each burst.

A Spiral Sample

Including the four bursts with newly identified host galaxies from this work, researchers have now localized the positions of 18 nearby fast radio bursts. What ties these burst-hosting galaxies together, if anything? As it turns out, they’re all spiral galaxies. That’s an intriguing result, but does it necessarily mean that fast radio bursts are more likely to come from spiral galaxies, or is it just easier to detect fast radio bursts from spiral galaxies?

Bhardwaj and collaborators explained that their sample selection is actually biased against bursts from spiral galaxies because these galaxies tend to have more signal-dispersing material than elliptical galaxies do, making spiral galaxies more likely to be eliminated by the dispersion measure cutoff. Given the typical ages of stars in spiral galaxies, the team suggests that the dominant formation pathway for fast radio bursts in the local universe is through core-collapse supernovae, which mark the explosive end of stars more than about eight times the mass of the Sun.

Bhardwaj’s team noted that this doesn’t mean all fast radio bursts must come from supernovae; because a small number of known bursts have arisen in unusual locations like globular clusters and non-star-forming spiral galaxies, delayed pathways like mergers of stellar remnants may be responsible for certain bursts.

Citation

“Host Galaxies for Four Nearby CHIME/FRB Sources and the Local Universe FRB Host Galaxy Population,” Mohit Bhardwaj et al 2024 ApJL 971 L51. doi:10.3847/2041-8213/ad64d1