Microphysics of Circumgalactic Turbulence Probed by Fast Radio Bursts and Quasars

Abstract

The circumgalactic medium (CGM) is poorly constrained at the sub-parsec scales relevant to turbulent energy dissipation and regulation of multi-phase structure. Fast radio bursts (FRBs) 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 $C_{\mathrm{n}}^2$, spectral index $\beta$, and dissipation scale $l_{\mathrm{i}}$. We use quasar observations of CGM turbulence at $\gtrsim$ pc scales to infer $C_{\mathrm{n}}^2$, finding it to be $10^{-16}\lesssim C_{\mathrm{n}}^2\lesssim 10^{-9}$ m${}^{-20/3}$ for hot ($T>10^6$ K) gas and $10^{-8}\lesssim C_{\mathrm{n}}^2\lesssim 10^{-4}$ m${}^{-20/3}$ for cool ($10^4\lesssim T\lesssim 10^5$ K) gas, depending on the gas sound speed and density. These values of $C_{\mathrm{n}}^2$ are much smaller than those inferred in the interstellar medium at similar physical scales. The resulting scattering delays from the hot CGM are negligible ($\ll 1$ $\mu$s at 1 GHz), but 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 $l_{\mathrm{i}}$, even if the CGM is not a significant scattering site. An initial comparison between quasar and FRB observations (albeit for different systems) suggests $l_{\mathrm{i}}\gtrsim 750$ km in $\sim 10^4$ 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 $l_{\mathrm{i}}$ may be comparable to the smallest cloud sizes ($\lesssim$ pc) inferred from photoionization modeling of quasar absorption lines.

21 pages, 9 figures, accepted for publication in ApJ

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