This fast radio burst shined a light on a galaxy’s mysterious gas halo

A distant blast of radio energy from one galaxy pierced the diffuse, gaseous halo around another, letting astronomers probe two cosmic oddities at once.

The brief, bright flare of a fast radio burst, or FRB, originated in a dim and distant galaxy, according to observations with a telescope array in the Australian outback (SN: 6/27/19). And by coincidence, the radio waves passed through another galaxy to reach Earth.

Looking at how the FRB changed as it zoomed through the nearer galaxy revealed that the galaxy had a surprisingly thin and calm halo of gas, researchers report September 26 in Science.

Most galaxies are surrounded by a haze called the circumgalactic medium, or CGM (SN: 7/12/18). Scientists think the CGM contains more mass than the galaxy’s stars and controls a galaxy’s life cycle. But because the CGM doesn’t give off much light of its own, it’s hard to study. Astronomers have relied on bright, distant objects like quasars to light up the CGM from behind, like headlights through fog.

Astrophysicist J. Xavier Prochaska of the University of California, Santa Cruz realized that FRBs could light up a CGM as well. Both quasars and FRBs emit light in a range of wavelengths, which are slowed along their path to Earth by charged particles en route. Because quasars shine continuously, it’s harder to tell how much the light is slowed. But the brief pulse of an FRB can reveal how much stuff is in the way. So there are characteristics of the CGM that FRBs can reveal directly, such as the gas’s density and magnetic fields, that longer-lasting quasars can’t.

“We can resolve these physical properties of the CGM that are pretty much impossible with other techniques,” Prochaska says. To do that, he just needed to find the right FRB.

He got lucky on November 12, 2018, when the Australian Square Kilometer Array Pathfinder detected an FRB pulse lasting less than 40 microseconds. Prochaska and colleagues tracked the FRB to a galaxy in the constellation Indus. That was a feat in itself — out of more than 60 FRBs previously found, only three others have been traced to their home galaxies so far (SN: 8/14/19).

At first, the team thought the FRB came from a bright galaxy about 4 billion light-years away. But the distance measured for the FRB was closer to 5 billion light-years. Prochaska realized there were two galaxies in a row — just the fortuitous lineup he had hoped for.

The foreground galaxy’s density and magnetization both were unexpectedly low, Prochaska and colleagues found. The density of CGM gas was less than 0.1 atoms per cubic centimeter, at least a factor of 10 lower than expected from previous studies. The magnetic field was less than 0.8 microgauss, a billion times weaker than a refrigerator magnet, suggesting that the gas doesn’t experience much turbulence.

The finding could suggest that the CGMs of some galaxies are less chaotic than previously thought, although more observations are needed to know for sure. Discovering more such FRBs that shine through CGMs could help. Prochaska says he expects to detect dozens more in the next few years.

“One of the great hopes for FRBs is using them as tools for probing everything” that lies along their paths to Earth, says astronomer James Cordes of Cornell University. “This is an excellent example of how FRBs can be used to probe basically anything that’s along the line of sight.”