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).