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

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 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 (10 Myr) and younger (10 Myr) stellar populations, respectively. Inclination raises the median DM measured for older populations (10 Myr) in the simulations by a factor of 2, but generally does not affect the younger stars deeply embedded in H II regions except in extreme edge-on cases (inclination 85^). 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 (10 Myr) and younger (10 Myr) stellar populations, respectively. FIRE galaxies tend to have higher DM values than cosmological simulations such as IllustrisTNG. 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.