Probing the CGM with FRB Dispersion and Scattering

Dylan Joe (Stanford University, KAVLI)

July 02, 2025

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Abstract: The CGM is thought to play an important role in mediating galaxy evolution and star formation; however, despite containing a significant fraction of the universe’s baryons, the physical state of the CGM remains relatively unconstrained. The ubiquitous observations of cool (T~10^4K) gas out to large radii complicate simple pictures of gas accretion and seem to point to the presence of structure in the gas down to small scales (subparsec) difficult to resolve in simulation. FRB scattering studies stand to shed light on the morphology of this cool gas on very small scales. But a problem has arisen: despite now thousands of FRBs detected, there has been no positive detection of any scattering of these sources outside of the Milky Way ISM or the host environment. What does this surprising absence of scattering from intervening CGM tell us about the nature of the cool gas? How can we detect scattering in the CGM or otherwise use FRBs as a probe of this mysterious medium?