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

الأصل · EN

We analyze the sources of free electrons that produce the large dispersion measures, DM ≈ 300-1600 (in units 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 IGM containing a fraction f IGM of cosmological baryons will produce DM = (935 cm⁻³ pc) f IGM h₇₀⁻¹ I(z), where I(z) = (2/3 Ωₘ)[{ Ωₘ(1+z)³ + ΩΛ }¹/² - 1]. A structured IGM of photoionized Ly-alpha absorbers in the cosmic web produces similar dispersion, modeled from the observed distribution, fb(N,z), of H I (Lya-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 Ly-alpha absorbers. Fluctuations in DM at the level ±10 cm⁻³ pc will arise from filaments and voids in the cosmic web.