2021ApJ...921..134S

Fast radio burst (FRB) dispersion measures (DMs) record the presence of ionized baryons that are otherwise invisible to other techniques enabling resolution of the matter distribution in the cosmic web. In this work, we aim to estimate the contribution to FRB 180924 DM from foreground galactic halos. Localized by ASKAP to a massive galaxy, this sightline is notable for an estimated cosmic web contribution to the DM (DM_cosmic =220 pc cm^–3), which is less than the average value at the host redshift (z = 0.3216) estimated from the Macquart relation (280 pc cm^–3). In the favored models of the cosmic web, this suggests few intersections with foreground halos at small impact parameters (<=100 kpc). To test this hypothesis, we carried out spectroscopic observations of the field galaxies within ∼1' of the sightline with VLT/MUSE and Keck/LRIS. Furthermore, we developed a probabilistic methodology that leverages photometric redshifts derived from wide-field DES and WISE imaging. We conclude that there is no galactic halo that closely intersects the sightline and also that the net DM contribution from halos DM_halos < 45 pc cm^–3 (95% c.l.). This value is lower than the DM_halos estimated from an "average" sightline (121 pc cm^–3) using the Planck ΛCDM model and the Aemulus halo mass function and reasonably explains its low DM_cosmic value. We conclude that FRB 180924 represents the predicted majority of sightlines in the universe with no proximate foreground galactic halos. Our framework lays the foundation for a comprehensive analysis of FRB fields in the near future.