2025ApJ...988...69O

The circumgalactic medium (CGM) is poorly constrained at the subparsec scales relevant to turbulent energy dissipation and regulation of multiphase structure. Fast radio bursts are sensitive to small-scale plasma density fluctuations, which can induce multipath propagation (scattering). The amount of scattering depends on the density fluctuation spectrum, including its amplitude ${C}_{{\rm{n}}}^{2}$, spectral index β, and dissipation scale l_i. We use quasar observations of CGM turbulence at ≳pc scales to infer ${C}_{{\rm{n}}}^{2}$, finding it to be $1{0}^{-16}\lesssim {C}_{{\rm{n}}}^{2}\lesssim 1{0}^{-9}$ m^–20/3 for hot (T > 10⁶ K) gas and $1{0}^{-8}\lesssim {C}_{{\rm{n}}}^{2}\lesssim 1{0}^{-4}$ m^–20/3 for cool (10⁴ ≲ T ≲ 10⁵ K) gas, depending on the gas sound speed and density. These values of ${C}_{{\rm{n}}}^{2}$ are much smaller than those inferred in the interstellar medium at similar physical scales. The resulting scattering delays from the hot CGM are negligible (≪1 μs at 1 GHz), but they are more detectable from the cool gas as either radio pulse broadening or scintillation, depending on the observing frequency and sightline geometry. Joint quasar-FRB observations of individual galaxies can yield lower limits on l_i, even if the CGM is not a significant scattering site. An initial comparison between quasar and FRB observations (albeit for different systems) suggests l_i ≳ 750 km in ∼10⁴ K gas in order for the quasar and FRB constraints to be consistent. If a foreground CGM is completely ruled out as a source of scattering along an FRB sightline, then l_i may be comparable to the smallest cloud sizes (≲pc) inferred from photoionization modeling of quasar absorption lines.