The astrophysical consequences of intervening galaxy gas on fast radio bursts

We adopt and analyse 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, parametrized as a double power-law; (ii) the incidence of DLAs with redshift (derived here), l(DLA)(z) = A + B arctan(z - C) with A = 0.236(-0.021)(+0.016), B = 0.168(-0.017)(+0.010) and C = 2.87(-0.13)(+0.17); and (iii) the electron density, parametrized as a log-normal deviate with mean 10(-2.6)cm(-3) and dispersion 0.3 dex. Synthesizing these results, we estimate that the average rest-frame dispersion measure (DM) from the neutral medium of a single, intersecting galaxy is DMDLANM = 0.25 pc cm(-3). Analysis of Al III and C II* absorption limits the putative warm ionized medium to contribute DMDLAWIM < 20 pc cm(-3). Given the low incidence of DLAs, we find that a population of FRBs at z = 2 will incur <(DM)over bar>(NM)(DLA) (z(FRB) = 2) = 0.01 pc cm(-3) on average, with a 99 per cent confidence level upper bound of 0.22 pc cm(-3). Assuming that turbulence of the interstellar medium (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(scatt) < 0.1 mas). The temporal broadening is also predicted to be small, tau(DLA) approximate to 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 25 ms is only approximately 0.1 per cent. 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.