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

TL;DR

This paper investigates how the structured intergalactic medium contributes to the dispersion measures of fast radio bursts at redshifts below 1.5, using models of cosmic web absorbers and cosmological simulations.

Contribution

It provides an analytic model linking FRB dispersion measures to the distribution of Ly-alpha absorbers and the baryon content of the IGM, enabling redshift estimation and baryon fraction calibration.

Findings

Estimated FRB redshifts range from 0.2 to 1.5 for typical IGM baryon fractions.

Structured IGM causes DM fluctuations of about ±10 cm$^{-3}$ pc due to cosmic web filaments and voids.

Model suggests future FRB surveys can calibrate IGM baryon content and Ly-alpha absorber distribution.

Abstract

We analyze the sources of free electrons that produce the large dispersion measures, DM $\approx 300-1600$ (in units cm$^{-3}$ pc), observed toward fast radio bursts (FRBs). Individual galaxies typically produce DM $\sim 25-60$ 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 $f_{\rm IGM}$ of cosmological baryons will produce DM $= (935~{\rm cm}^{-3}~{\rm pc}) f_{\rm IGM} \, h_{70}^{-1} I(z)$, where $I(z) = (2/3 \Omega_m)[ \{ \Omega_m(1+z)^3 + \Omega_{\Lambda} \}^{1/2} - 1 ]$. A structured IGM of photoionized Ly-alpha absorbers in the cosmic web produces similar dispersion, modeled from the observed distribution, $f_b(N,z)$, 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 $z_{\rm FRB} \approx 0.2-1.5$ for an IGM containing a significant baryon fraction, $f_{\rm IGM} = 0.6\pm0.1$. 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 $\pm10$ cm$^{-3}$ pc will arise from filaments and voids in the cosmic web.