Objects May Be Closer Than They Appear: Significant Host Galaxy Dispersion Measures of Fast Radio Bursts in Zoom-in Simulations

TL;DR

This study uses detailed simulations to quantify how host galaxy environments contribute to the dispersion measures of fast radio bursts, revealing that these contributions can be larger than previously thought and impact FRB distance estimates.

Contribution

It provides the first detailed simulation-based analysis of host galaxy DM contributions for FRBs, highlighting the importance of galaxy inclination and star age.

Findings

Host galaxy DMs are higher at kinematic disturbances at z=1.

Inclination significantly increases DMs for older stellar populations.

FIRE galaxies show higher DMs than other cosmological simulations.

Abstract

We investigate the contribution of host galaxies to the overall Dispersion Measures (DMs) for Fast Radio Bursts (FRBs) using the Feedback in Realistic Environments (FIRE-2) cosmological zoom-in simulation suite. We calculate DMs from every star particle in the simulated L* galaxies by ray-tracing through their multi-phase interstellar medium (ISM), summing the line-of-sight free thermal electron column for all gas elements within $\pm$20 kpc of the galactic mid-plane. At $z=0$, we find average (median) host-galaxy DMs of 74 (43) and 210 (94) pc cm$^{-3}$ for older ($\gtrsim$10 Myr) and younger ($\lesssim$10 Myr) stellar populations, respectively. Inclination raises the median DM measured for older populations ($\gtrsim$10 Myr) in the simulations by a factor of $\sim$2, but generally does not affect the younger stars deeply embedded in H{\small II} regions except in extreme edge-on cases (inclination $\gtrsim 85^\circ$). In kinematically disturbed snapshots ($z = 1$ in FIRE), the average (median) host-galaxy DMs are higher: 80 (107) and 266 (795) pc cm$^{-3}$ for older ($\gtrsim$10 Myr) and younger ($\lesssim$10 Myr) stellar populations, respectively. FIRE galaxies tend to have higher DM values than cosmological simulations such as IllustrisTNG\rev{, with larger tails in their distributions to high DMs}. As a result, FRB host galaxies may be closer (lower redshift) than previously inferred. Furthermore, constraining host-galaxy DM distributions may help significantly constrain FRB progenitor models.