Cosmology-independent Estimate of the Fraction of Baryon Mass in the IGM from Fast Radio Burst Observations

Abstract

The excessive dispersion measure (DM) of fast radio bursts (FRBs) has been proposed to be a powerful tool to study intergalactic medium (IGM) and to perform cosmography. One issue is that the fraction of baryons in the IGM, f <inf>IGM</inf>, is not properly constrained. Here, we propose a method of estimating f <inf>IGM</inf> using a putative sample of FRBs with the measurements of both DM and luminosity distance d <inf>L</inf>. The latter can be obtained if the FRB is associated with a distance indicator (e.g., a gamma-ray burst or a gravitational-wave event), or the redshift z of the FRB is measured and d <inf>L</inf> at the corresponding z is available from other distance indicators (e.g., SNe Ia) at the same redshift. As d <inf>L</inf>/DM essentially does not depend on cosmological parameters, our method can determine f <inf>IGM</inf> independent of cosmological parameters. We parameterize f <inf>IGM</inf> as a function of redshift and model the DM contribution from a host galaxy as a function of star formation rate. Assuming f <inf>IGM</inf> has a mild evolution with redshift with a functional form and by means of Monte Carlo simulations, we show that an unbiased and cosmology-independent estimate of the present value of f <inf>IGM</inf> with a ∼12% uncertainty can be obtained with 50 joint measurements of d <inf>L</inf> and DM. In addition, such a method can also lead to a measurement of the mean value of DM contributed from the local host galaxy.