An FRB Sent Me a DM: Constraining the Electron Column of the Milky Way Halo with Fast Radio Burst Dispersion Measures from CHIME/FRB

TL;DR

This study uses fast radio burst dispersion measures from CHIME/FRB to constrain the electron content of the Milky Way's halo, providing new limits on its ionized gas distribution and informing galaxy formation models.

Contribution

It introduces observational constraints on the Milky Way halo's electron column density using FRB data, testing various gas density profiles and feedback effects.

Findings

Galactic DM contribution ranges from 87.8 to 141 pc cm^-3 at high latitudes.

Halo models with high gas mass overestimate the DM contribution compared to observations.

Feedback processes significantly influence the halo gas distribution and DM estimates.

Abstract

The CHIME/FRB project has detected hundreds of fast radio bursts (FRBs), providing an unparalleled population to probe statistically the foreground media that they illuminate. One such foreground medium is the ionized halo of the Milky Way (MW). We estimate the total Galactic electron column density from FRB dispersion measures (DMs) as a function of Galactic latitude using four different estimators, including ones that assume spherical symmetry of the ionized MW halo and ones that imply more latitudinal-variation in density. Our observation-based constraints of the total Galactic DM contribution for $|b|\geq 30^\circ$, depending on the Galactic latitude and selected model, span 87.8 - 141 pc cm^-3. This constraint implies upper limits on the MW halo DM contribution that range over 52-111 pc cm^-3. We discuss the viability of various gas density profiles for the MW halo that have been used to estimate the halo's contribution to DMs of extragalactic sources. Several models overestimate the DM contribution, especially when assuming higher halo gas masses (~ 3.5 x 10^12 solar masses). Some halo models predict a higher MW halo DM contribution than can be supported by our observations unless the effect of feedback is increased within them, highlighting the impact of feedback processes in galaxy formation.