Extragalactic dispersion measures of fast radio bursts
  Fast radio bursts show large dispersion measures, much larger than the
Galactic dispersion measure foreground. Therefore,they evidently have an
extragalactic origin. We investigate possible contributions to the dispersion
measure from host galaxies. We simulate the spatial distribution of fast radio
bursts and calculate the dispersion measures along the sightlines from fast
radio bursts to the edge of host galaxies by using the scaled NE2001 model for
thermal electron density distributions. We find that contributions to the
dispersion measure of fast radio bursts from the host galaxy follow a skew
Gaussian distribution. The peak and the width at half maximum of the dispersion
measure distribution increase with the inclination angle of a spiral galaxy, to
large values when the inclination angle is over $70\degr$. The largest
dispersion measure produced by an edge-on spiral galaxy can reach a few
thousand pc~cm$^{-3}$, while the dispersion measures from dwarf galaxies and
elliptical galaxies have a maximum of only a few tens of pc~cm$^{-3}$. Notice,
however, that additional dispersion measures of tens to hundreds of
pc~cm$^{-3}$ can be produced by high density clumps in host galaxies.
Simulations that include dispersion measure contributions from the Large
Magellanic Cloud and the Andromeda Galaxy are shown as examples to demonstrate
how to extract the dispersion measure from the intergalactic medium.
