Large host-galaxy dispersion measure of fast radio bursts

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 ($⟨\mathrm{D}{\mathrm{M}}_{\mathrm{H}\mathrm{G},\mathrm{l}\mathrm{o}\mathrm{c}}⟩$) 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. $\left\langle{\rm DM_{HG,loc}}\right\rangle \sim 270~\rm{pc~cm^{-3}}$ with a large dispersion. A relatively large $\mathrm{D}{\mathrm{M}}_{\mathrm{H}\mathrm{G}}$ 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 $\mathrm{D}{\mathrm{M}}_{\mathrm{H}\mathrm{G},\mathrm{l}\mathrm{o}\mathrm{c}}\sim 210\mathrm{p}\mathrm{c}\mathrm{c}{\mathrm{m}}^{-3}$). The large host galaxy DM may be contributed by the 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 (MW)-like, consistent with the detected host of FRB 121102. We also discuss the possibility of having a near-source supernova remnant (SNR), pulsar wind nebula (PWN) or HII region that gives a significant contribution to the observed $\mathrm{D}{\mathrm{M}}_{\mathrm{H}\mathrm{G}}$.