Oral Presentation

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

Author(s):

Presenter: Matt Orr (Flatiron Institute/Center for Computational Astrophysics)

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$20 kpc of the galactic mid-plane. At z=0, we find average host-galaxy DMs of 74 and 210 pc cm$^{-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 $\sim$0.3 dex, 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 host-galaxy DMs are higher: 80 and 266 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. As a result, we generally find that FRB host galaxies may be closer (lower redshift) than previously inferred, given higher host-galaxy contributions to the DM. Furthermore, constraining host-galaxy DM distributions may help significantly constrain FRB progenitor models to younger populations.