Microphysics of Circumgalactic Turbulence Probed by Fast Radio Bursts and Quasars

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 Cn2 , spectral index β, and dissipation scale li. We use quasar observations of CGM turbulence at ≳pc scales to infer Cn2 , finding it to be 10−16≲Cn2≲10−9 m−20/3 for hot (T > 106 K) gas and 10−8≲Cn2≲10−4 m−20/3 for cool (104 ≲ T ≲ 105 K) gas, depending on the gas sound speed and density. These values of Cn2 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 li, even if the CGM is not a significant scattering site. An initial comparison between quasar and FRB observations (albeit for different systems) suggests li ≳ 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 li may be comparable to the smallest cloud sizes (≲pc) inferred from photoionization modeling of quasar absorption lines.