# Probing Galactic Halos with Fast Radio Bursts

## TL;DR

This paper explores how fast radio bursts can be used to probe the ionized gas in galactic halos, providing new insights into the distribution of baryonic matter around galaxies.

## Contribution

It combines observational constraints and models to estimate halo gas contributions to dispersion measures and demonstrates the potential of FRB surveys to detect halo gas in the Local Group.

## Key findings

- Galactic halo contributes ~50-80 pc/cm^3 to DM.
- M31's halo detectable by upcoming FRB surveys.
- Simulations show FRBs can resolve baryonic content in halos.

## Abstract

The precise localization (<1'') 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 halos (aka halo gas) on DM observations in current and forthcoming FRB surveys. Combining constraints from observations of the high velocity clouds, OVII absorption, and the DM to the Large Magellanic Cloud with hydrostatic models of halo gas, we estimate that our Galactic halo will contribute ${\rm DM}_{\rm MW,halo} \approx 50-80 \rm pc/cm^{-3}$ 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 halos by analyzing simulated sightlines through the CASBaH survey. All of the code and data products of our analysis are available at https://github.com/FRBs.