2017ApJ...839L..25Y

Fast radio bursts (FRBs) have excessive dispersion measures (DMs) and an all-sky distribution, which point toward an extragalactic or even a cosmological origin. We develop a method to extract the mean host galaxy DM (<DM_HG,loc>) and the characterized luminosity (L) of FRBs using the observed DM-flux data, based on the assumption of a narrow luminosity distribution. Applying Bayesian inference to the data of 21 FRBs, we derive a relatively large mean host DM, i.e., <DM_HG,loc>∼270 pc cm^–3 with a large dispersion. A relatively large DM_HG of FRBs is also supported by the millisecond scattering times of some FRBs and the relatively small redshift z = 0.19273 of FRB 121102 (which gives DM_HG,loc∼210 pc cm^–3). The large host galaxy DM may be contributed by the interstellar medium (ISM) or a near-source plasma in the host galaxy. If it is contributed by the ISM, the type of the FRB host galaxies would not be Milky Way-like, consistent with the detected host of FRB 121102. We also discuss the possibility of having a near-source supernova remnant, pulsar wind nebula, or H II region that gives a significant contribution to the observed DM_HG.