2018ApJ...852L..11S

We analyze the sources of free electrons that produce the large dispersion measures, DM~300–1600 (in units of cm^–3 pc), observed toward fast radio bursts (FRBs). Individual galaxies typically produce DM∼25–60 cm^–3 pc 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 f_IGM of cosmological baryons will produce DM=(935 cm^–3 pc)f_IGM h₇₀^–1I(z), where I(z)=(2/3Ω_m)[Ω_m(1+z)³+Ω_Λ^1/2-1]. A structured IGM of photoionized Lyα absorbers in the cosmic web produces similar dispersion, modeled from the observed distribution, f_b(N,z), of H I (Lyα-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 z_FRB ~0.2–1.5 for an IGM containing a significant baryon fraction, f_IGM=0.6 ±0.1. Future surveys of the statistical distribution, DM(z), of FRBs identified with specific galaxies and redshifts can be used to calibrate the IGM baryon fraction and distribution of Lyα absorbers. Fluctuations in DM at the level ±10 cm^–3 pc will arise from filaments and voids in the cosmic web.