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Fast Radio Burst (FRB) dispersion measures (DMs) record the presence ofionized baryons that are otherwise invisible to other techniques enablingresolution of the matter distribution in the cosmic web. In this work, we aimto estimate the contribution to FRB 180924 DM from foreground galactic halos.Localized by ASKAP to a massive galaxy, this sightline is notable for anestimated cosmic web contribution to the DM ($\rm DM_{cosmic} =220~pc~cm^{-3}$), which is less than the average value at the host redshift($\rm z = 0.3216$) estimated from the Macquart relation ($280~\rm pc~cm^{-3}$).In the favored models of the cosmic web, this suggests few intersections withforeground halos at small impact parameters ($\lesssim 100$ kpc). To test thishypothesis, we carried out spectroscopic observations of the field galaxieswithin $\sim$1' of the sightline with VLT/MUSE and Keck/LRIS. Furthermore, wedeveloped a probabilistic methodology that leverages photometric redshiftsderived from wide-field DES and WISE imaging. We conclude that there is nogalactic halo that closely intersects the sightline and also that the net DMcontribution from halos, $\rm DM_{halos}< 45~pc~cm^{-3}$ (95 % c.l.). Thisvalue is lower than the $\rm DM_{halos}$ estimated from an "average" sightline($121~\rm pc~cm^{-3}$) using the Planck $\Lambda CDM$ model and the Aemulushalo mass function and reasonably explains its low $\rm DM_{cosmic}$ value. Weconclude that FRB 180924 represents the predicted majority of sightlines in theuniverse with no proximate foreground galactic halos. Our framework lays thefoundation for a comprehensive analysis of FRB fields in the near future.

Estimating the contribution of foreground halos to the FRB 180924 dispersion measure