Astronomers Gain Insights Into Galactic Halos Thanks To Rare Fast Radio Burst

This illustration shows the radio signal from the fast radio burst FRB 181112 passing through the halo of a foreground galaxy on its way toward the telescopes that detected it on Earth. J. Josephides/Centre for Astrophysics and Supercomputing, Swinburne University of Technology

Galaxies in the modern universe are surrounded by huge halos of gas and dark matter that can extend hundreds of thousands of light-years across. The properties of such halos are difficult to measure since the gas is extremely diffuse, but now astronomers have received help from an unexpected event, a fast radio burst (FRB).

FRBs are incredible releases of energy that only last a fraction of a second and are detected in radio waves. The origin of most of these events is a complete mystery and we have struggled to detect them quickly enough to localize them in the sky. Lately, researchers have been able to pinpoint their origin with more precision.

As reported in the journal Science, FRB 181112’s position is known with high precision and its powerful radio waves passed through the halo of a foreground galaxy before reaching Earth in November 2018. These two characteristics gave researchers a chance to find out more about halos. The gas surrounding this galaxy is a lot less turbulent and has a lot less magnetism than previous observations suggested.

“The signal from the fast radio burst exposed the nature of the magnetic field around the galaxy and the structure of the halo gas. The study proves a new and transformative technique for exploring the nature of galaxy halos,” Professor J. Xavier Prochaska, from UC Santa Cruz, said in a statement. “This galaxy’s halo is surprisingly tranquil. The radio signal was largely unperturbed by the galaxy, which is in stark contrast to what previous models predict would have happened to the burst.”

FRB 181112 consisted of several pulses, each lasting less than 40 microseconds. A pulse's length depends on the density of the material it crosses, so the halo of the galaxy must be very diffuse, fewer than 100 atoms in a cubic meter.

“Like the shimmering air on a hot summer’s day, the tenuous atmosphere in this massive galaxy should warp the signal of the fast radio burst," explained co-author Jean-Pierre Macquart, an astronomer at the International Center for Radio Astronomy Research at Curtin University, Australia. "Instead we received a pulse so pristine and sharp that there is no signature of this gas at all.” 

While the research is exciting, it is about one single galaxy. The team is unsure if this object is typical or peculiar. They plan to use more FRBs to study tens if not hundreds of these object to better understand the evolution of gas halos.

Halos are where the material thrown out by active supermassive black holes end up. Understanding the halos’ properties can tell us a lot about how galaxies evolve.

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