2019ApJ...876..146L

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_IGM, is not properly constrained. Here, we propose a method of estimating f_IGM using a putative sample of FRBs with the measurements of both DM and luminosity distance d_L. 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_L at the corresponding z is available from other distance indicators (e.g., SNe Ia) at the same redshift. As d_L/DM essentially does not depend on cosmological parameters, our method can determine f_IGM independent of cosmological parameters. We parameterize f_IGM as a function of redshift and model the DM contribution from a host galaxy as a function of star formation rate. Assuming f_IGM 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_IGM with a ∼12% uncertainty can be obtained with 50 joint measurements of d_L 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.