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_=220pc/cm^3^), which is less than the average value at the host redshift (z=0.3216) estimated from the Macquart relation (280pc/cm^3^). In the favored models of the cosmic web, this suggests few intersections with foreground halos at small impact parameters (<=100kpc). 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_<45pc/cm^3^ (95% c.l.). This value is lower than the DMhalos estimated from an "average" sightline (121pc/cm^3^) using the Planck {Lambda}CDM model and the Aemulus halo mass function and reasonably explains its low DMcosmic 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.

Cone search capability for table J/ApJ/921/134/table1 (Catalog of photometry from DES and WISE for galaxies in the FRB 180924 field)

Cone search capability for table J/ApJ/921/134/table2 (MUSE and LRIS sources with unambiguous redshifts)