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Cordes and Chatterjee
have an article in the upcoming Annual Reviews of Astronomy & Astrophysics
entitled "Fast Radio Bursts: An Extragalactic Enigma".
The figure above shows Dispersion Measures (DMs)
plotted against Galactic latitude for FRBs and for
pulsars in the Milky Way and in the Large and Small Magellanic Clouds (LMC, SMC).
They note that two conclusions can be drawn from the figure.
First, the DMs of all FRBs with galactic latitudes greater than 10 degrees
from the Galactic plane are much larger than
the outer envelope for Galactic pulsars. An
extragalactic population of FRBs would appear just this way if the total DM includes a
large extragalactic contribution.
Second, the DMs of FRBs cover a range from about 100 to 2600 pc/cm^3.
This is comparable to the range for pulsars, that are
clearly due to the inter-stellar medium (ISM) of our Galaxy and, in a few cases, the ISM in the Magellanic clouds.
The extragalactic contributions for the smallest DMs are equal to those expected from a
dwarf galaxy, as indicated by the excesses seen for pulsars in the Magellanic
clouds. The largest DMs are comparable to those expected from either a long path through
the inter-galactic medium or a galaxy disk, from a galactic center like that of the Milky Way, or from a young
supernova remnant. Ionized gas in galaxies is therefore a plausible source for
some or most of the extragalactic part of DM.
More details are given in
the preprint to the paper.
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