Astronomers Used a Fast Radio Burst to Probe the Structure of the Milky Way

In the past decade and a half, hundreds of Fast Radio Bursts (FRBs) have been detected by astronomers. These transient energetic bursts occur suddenly, typically last for just a few milliseconds, and are rarely seen again (except in the rare case of repeating bursts). While astronomers are still not entirely sure what causes this phenomenon, FRBs have become a tool for astronomers hoping to map out the cosmos. Based on the way radio emissions are dispersed as they travel through space, astronomers can measure the structure and distribution of matter in and around galaxies.

Using the Deep Synoptic Array (DSA) at the Owens Valley Radio Observatory (OVRO), a team of astronomers from Caltech and Cornell University used an intense FRB from a nearby galaxy to probe the halo of hot gas that surrounds the Milky Way. Their results show that our galaxy has significantly less visible (“baryonic” or “normal”) matter than previously expected. These findings support theories that matter is regularly ejected from our galaxy due to stellar winds, supernovae, and accreting supermassive black holes (SMBHs).

The research was conducted by the Deep Synoptic Array (DSA) team, made up of researchers from Caltech’s Cahill Center for Astronomy and Astrophysics, the Owens Valley Radio Observatory (OVRO), the Department of Astronomy and the Cornell Center for Astrophysics and Planetary Science at Cornell University. The paper that describes their findings, which was recently submitted to The Astrophysical Journal, is the latest in a series of results from Caltech’s DSA – a collection of radio dishes funded by the National Science Foundation (NSF).

One of the biggest challenges in studying FRBs has been identifying their place of origin, which is extremely difficult given how short-lived most are. Knowing where these mysterious bursts come from allows astronomers to monitor the source (in case they happen to repeat) and helps them to narrow down what could be triggering the intense flashes. In addition, identifying their locations is essential for using FRBs to study how much matter is distributed across the Universe. To date, astronomers have only been able to trace 21 events back to their galaxies.

The DSA-110 was purpose-built to detect and localize FRBs using 110 × 4.65-m dishes that continuously survey the sky at frequencies between 1280 – 1530 MHz. The array was commissioned in February 2022 and will spend the next three years tracing 300 FRBs to regions smaller than 3 arcseconds (<1/1000th of a degree) – it has discovered and pinpointed the locations of 30 new FRBs to date. Vikram Ravi, an assistant professor of astronomy at Caltech who leads the science team for DSA, presented the results at the 241st Meeting of the American Astronomical Society (AAS) – which ran from January 8th to 12th in Seattle.

The early results (as reported in the paper) showed that our Milky Way has less matter than expected and raised new questions about what causes them. Previous studies, like the FRB detected in 2020 with the help of Caltech’s STARE2 project, supported the theory that FRBs are likely caused by stellar remnants with extreme magnetic fields (magnetars). However, the new DSA observations show that FRBs have diverse origins (including older galaxies within rich galaxy clusters). The results also suggest that if FRBs are emitted by magnetars, they are likely formed through multiple unknown pathways. As Ravi said in a Caltech press release:

“We were puzzled at first about why we were discovering so many FRBs. But it comes down to careful engineering of the antennas and receivers, and the software pipelines. We now rarely miss a thing. Magnetars like those in the Milky Way are formed during episodes of intense star formation. To find FRBs from galaxies that have mostly stopped forming stars was surprising. The DSA gathers and processes enormous amounts of data all the time. The data rate is equivalent to watching 28,000 Netflix movies at once.”

The DSA will become even more powerful in the future as all 110 dishes come online (only 63 are currently in operation). Caltech and its partners also plan to build an array of 2,000 radio dishes to create the DSA-2000, which will be the most powerful radio survey telescope ever built. The project would process data at a rate equivalent to roughly 20 percent of today’s global internet traffic (several dozen exabytes). It will also detect an estimated one billion new radio sources, including 40,000 new FRBs (100 times more than we’ve detected so far).

Caltech professor Gregg Hallinan, the director of the Owens Valley Radio Observatory, will be the principal investigator of DSA-2000. “The DSA-2000 will build upon progress with the DSA and revolutionize radio astronomy,” he said.

Further Reading: Caltech