Large Host-galaxy Dispersion Measure of Fast Radio Bursts

Abstract

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 (aDM<inf>HG,loc</inf>n) 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 FRB<inf>s</inf>, we derive a relatively large mean host DM, i.e., aDM<inf>HG,loc</inf>n ∼ 270 pc cm^-3 with a large dispersion. A relatively large DMHG 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<inf>HG</inf>.