The Dispersion of Fast Radio Bursts from a Structured Intergalactic Medium at Redshifts z < 1.5

Abstract

We analyze the sources of free electrons that produce the large dispersion measures, (in units of cm −3 pc), observed toward fast radio bursts (FRBs). Individual galaxies typically produce from ionized gas in their disk, disk-halo interface, and circumgalactic medium. Toward an FRB source at redshift z , a homogeneous intergalactic medium (IGM) containing a fraction of cosmological baryons will produce , where . A structured IGM of photoionized Ly α absorbers in the cosmic web produces similar dispersion, modeled from the observed distribution, , of H i (Ly α -forest) absorbers in column density and redshift with ionization corrections and scaling relations from cosmological simulations. An analytic formula for DM( z ) applied to observed FRB dispersions suggests that for an IGM containing a significant baryon fraction, . Future surveys of the statistical distribution, DM( , of FRBs identified with specific galaxies and redshifts can be used to calibrate the IGM baryon fraction and distribution of Ly α absorbers. Fluctuations in DM at the level ±10 cm −3  pc will arise from filaments and voids in the cosmic web.