The Astrophysical Consequences of Intervening Galaxy Gas on Fast Radio
  Bursts
  We adopt and analyze results on the incidence and physical properties of
damped Ly$\alpha$ systems (DLAs) to predict the astrophysical impact of gas in
galaxies on observations of Fast Radio Bursts (FRBs). Three DLA measures form
the basis of this analysis: (i) the HI column density distribution,
parameterized as a double power-law; (ii) the incidence of DLAs with redshift
(derived here), $\ell(z)=A+B \arctan(z-C)$ with $A=0.236_{-0.021}^{+0.016},
B=0.168_{-0.017}^{+0.010}, C=2.87_{-0.13}^{+0.17}$ and (iii) the electron
density, parameterized as a log-normal deviate with mean $10^{-2.6} cm^{-3}$
and dispersion 0.3dex. Synthesizing these results, we estimate that the average
rest-frame dispersion measure from the neutral medium of a single, intersecting
galaxy is DM$^{NM}_{DLA}=0.25$ pc/cm^3. Analysis of AlIII and CII* absorption
limits the putative warm ionized medium to contribute
DM$^{WIM}_{DLA}<20$pc/cm^3. Given the low incidence of DLAs, we find that a
population of FRBs at z=2 will incur DM(z=2)=0.01 pc/cm^3 on average, with a
99% c.l. upper bound of 0.22 pc/cm^3. Assuming that turbulence of the ISM in
external galaxies is qualitatively similar to our Galaxy, we estimate that the
angular broadening of an FRB by intersecting galaxies is negligible
($\theta<0.1$mas). The temporal broadening is also predicted to be small, $\tau
\approx 0.3$ms for a z=1 galaxy intersecting a z=2 FRB for an observing
frequency of $\nu=1$GHz. Even with $\nu=600$MHz, the fraction of sightlines
broadened beyond 25ms is only approximately 0.1%. We conclude that gas within
the ISM of intervening galaxies has a minor effect on the detection of FRBs and
their resultant DM distributions. Download the repository at
https://github.com/FRBs/FRB to repeat and extend the calculations presented
here.
