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Radio wave scattering can cause severe reductions in detection sensitivity for surveys of Galactic and extragalactic fast ($\sim$ms duration) transients. While Galactic sources like pulsars are subject to scattering in the Milky Way interstellar medium (ISM), extragalactic fast radio bursts (FRBs) can also experience scattering in their host galaxies and other galaxies intervening their lines-of-sight. We assess Galactic and extragalactic scattering horizons for fast radio transients using a combination of NE2001 to model the dispersion measure (DM) and scattering time ($\tau$) contributed by the Milky Way, and independently constructed electron density models for other galaxies' ISMs and halos that account for different galaxy morphologies, masses, densities, and strengths of turbulence. For FRB source redshifts $z_{\rm s} \lesssim 1$, an all-sky, isotropic FRB population has values of $\tau$ ranging between $\sim 1\ \mu$s and $\sim 2$ ms at 1 GHz (observer frame) that are dominated by host galaxies. For a hypothetical, high-redshift ($z_{\rm s}\sim5$) FRB population, $\tau$ ranges from $\sim 0.01 - 100$s of ms at 1 GHz, and is largely dominated by intervening galaxies. About $20\%$ of these high-redshift FRBs are predicted to have $\tau > 5$ ms at 1 GHz (observer frame), and $\gtrsim 40\%$ of FRBs between $z_{\rm s} \sim 0.5 - 5$ are predicted to have $\tau \gtrsim 1$ ms for $\nu\leq 800$ MHz. The percentage of FRBs selected against from scattering may be substantially larger because our scattering predictions are conservative compared to localized FRBs, and if circumgalactic turbulence causes density fluctuations larger than those observed from nearby halos., Comment: 24 pages, 14 figures, submitted to ApJ

Radio wave scattering can cause severe reductions in detection sensitivity for surveys of Galactic and extragalactic fast ($\sim$ms duration) transients. While Galactic sources like pulsars undergo scattering in the Milky Way interstellar medium (ISM), extragalactic fast radio bursts (FRBs) can also experience scattering in their host galaxies and other galaxies intervening their lines-of-sight. We assess Galactic and extragalactic scattering horizons for fast radio transients using a combination of NE2001 to model the dispersion measure (DM) and scattering time ($\tau$) contributed by the Galactic disk, and independently constructed electron density models for the Galactic halo and other galaxies' ISMs and halos that account for different galaxy morphologies, masses, densities, and strengths of turbulence. For source redshifts $0.5\leq z_{\rm s}\leq1$, an all-sky, isotropic FRB population has simulated values of $\tau(1\rm~GHz)$ ranging from $\sim1~\mu$s to $\sim2$ ms ($90\%$ confidence, observer frame) that are dominated by host galaxies, although $\tau$ can be $\gg2$ ms at low Galactic latitudes. A population at $z_{\rm s}=5$ has $0.01\lesssim\tau\lesssim300$ ms at 1 GHz ($90\%$ confidence), dominated by intervening galaxies. About $20\%$ of these high-redshift FRBs are predicted to have $\tau>5$ ms at 1 GHz (observer frame), and $\gtrsim40\%$ of FRBs between $z_{\rm s}\sim0.5-5$ have $\tau\gtrsim1$ ms for $\nu\leq 800$ MHz. Our scattering predictions may be conservative if scattering from circumsource environments is significant, which is possible under specific conditions. The percentage of FRBs selected against from scattering could also be substantially larger than we predict if circumgalactic turbulence causes more small-scale ($\ll1$ au) density fluctuations than observed from nearby halos., Comment: 27 pages, 16 figures, accepted to ApJ