A detection of circumgalactic gas with fast radio bursts
  Galaxies and groups of galaxies exist in dark-matter halos filled with
diffuse gas. The diffuse gas represents up to 80\% of the mass in luminous
matter within the halos (1,2), and is difficult to detect because of its low
density (particle number densities of $\lesssim10^{-4}$\,cm$^{-3}$) and high
temperature (mostly greater than $10^{6}$\,K). The spatial distribution and
total mass of this material determines, and is influenced by, the evolution of
galaxies and galaxy groups (3-5). Existing observational constraints on these
quantities are limited by sensitivity, and the necessity to accurately model
the ionization fraction, metal content, and pressure of the gas (6-8). Here we
report the detection of diffuse gas associated with nearby galaxies using the
dispersion measures (DMs) of extragalactic fast radio bursts (FRBs). FRB DMs
provide direct measurements of the total ionized-gas contents along their
sightlines. Out of a sample of 474 distant FRBs from the CHIME/FRB Catalog 1
(9), we identify a sample of 24 events that likely intersect the dark-matter
halos of galaxies in the local Universe ($<40$\,Mpc). This subset of FRBs has
an excess mean DM of $200\pm100$\,pc\,cm$^{-3}$ over those that do not
intersect nearby galaxies. The excess is larger than expected for the diffuse
gas surrounding isolated galaxies, but may be explained by additional
contributions from gas surrounding galaxy groups, including from the Local
Group. This result demonstrates the predicted ability of FRBs to be used as
sensitive, model-independent measures of the diffuse-gas contents of
dark-matter halos (10-13).
