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Based on https://arxiv.org/abs/2504.19562

The size of cool gas clouds in the circumgalactic medium (CGM) of galaxies holds information about the physical processes that shape this multiphase environment. We explore constraints on the size of cool gas clouds obtainable from the presence, or lack thereof, of refractive scattering in fast radio bursts (FRBs). We find that the bulk of low-redshift cool CGM gas, constrained to have densities of ne <∼10−2 cm−3, likely cannot produce two refractive images and, hence, scattering. It is only for extremely small cloud sizes ≲ 0.1 pc (about a hundred times smaller than the so-called shattering scale) that such densities could result in detectable scattering. Dense ne ≳ 0.1 cm−3 gas with shattering-scale cloud sizes is more likely to inhabit the inner several kiloparsecs of the low-redshift CGM: such clouds would result in multiple refractive images and large scattering times ≳ 1 − 10 ms, but a small fraction FRB sightlines are likely to be affected. We argue that such large scattering times from an intervening CGM would be a signature of sub- parsec clouds, even if diffractive scattering from turbulence contributes to the overall scattering.