Probing Galactic haloes with fast radio bursts

The precise localization (<1 arcsec) of multiple fast radio bursts (FRBs) to z > 0.1 galaxies has confirmed that the dispersion measures (DMs) of these enigmatic sources afford a new opportunity to probe the diffuse ionized gas around and in between galaxies. In this manuscript, we examine the signatures of gas in dark matter haloes (aka halo gas) on DM observations in current and forthcoming FRB surveys. Combining constraints from observations of the high-velocity clouds, O VII absorption, and the DM to the Large Magellanic Cloud with hydrostatic models of halo gas, we estimate that our Galactic halo will contribute {DM_MW,halo}≈ 50-80 pc cm^{-3} from the Sun to 200 kpc independent of any contribution from the Galactic ISM. Extending analysis to the Local Group, we demonstrate that M31's halo will be easily detected by high-sample FRB surveys (e.g. CHIME) although signatures from a putative Local Group medium may compete. We then review current empirical constraints on halo gas in distant galaxies and discuss the implications for their DM contributions. We further examine the DM probability distribution function of a population of FRBs at z ≫ 0 using an updated halo mass function and new models for the halo density profile. Lastly, we illustrate the potential of FRB experiments for resolving the baryonic fraction of haloes by analysing simulated sightlines through the CASBaH survey. All of the codes and data products of our analysis are available at https://github.com/FRBs.