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