The low density and magnetization of a massive galaxy halo exposed by a fast radio burst

TL;DR

This study uses a fast radio burst to probe the halo gas of a galaxy, revealing it to be low in density and magnetization, challenging previous assumptions about such halos.

Contribution

First direct measurement of the physical properties of halo gas in a massive galaxy using a precisely localized fast radio burst.

Findings

Halo gas has low magnetization and turbulence.

Halo gas is predominantly diffuse and low density.

Results challenge previous models of galaxy halo composition.

Abstract

Present-day galaxies are surrounded by cool and enriched halo gas extending to hundreds of kiloparsecs. This halo gas is thought to be the dominant reservoir of material available to fuel future star formation, but direct constraints on its mass and physical properties have been difficult to obtain. We report the detection of a fast radio burst (FRB 181112) with arcsecond precision, which passes through the halo of a foreground galaxy. Analysis of the burst shows the halo gas has low net magnetization and turbulence. Our results imply predominantly diffuse gas in massive galactic halos, even those hosting active supermassive black holes, contrary to some previous results.