The Astrophysical Consequences of Intervening Galaxy Gas on Fast Radio Bursts

Abstract

We adopt and analyze results on the incidence and physical properties of damped Ly$\hat{I}\pm$ 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 $10^{-2.6}c{m}^{-3}$ 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${}_{DLA}^{NM}=0.25$ pc/cm^3. Analysis of AlIII and CII* absorption limits the putative warm ionized medium to contribute DM${}_{DLA}^{WIM}<20$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 ($\hat{I}¸<0.1$mas). The temporal broadening is also predicted to be small, $\ddot{I}„\approx 0.3$ms for a z=1 galaxy intersecting a z=2 FRB for an observing frequency of $\hat{I}½=1$GHz. Even with $\hat{I}½=600$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.

9 pages, 8 figures; See https://github.com/FRBs/FRB for related code; Accepted to MNRAS