Estimating the contribution of foreground halos to the FRB 180924
  dispersion measure
  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 ($\rm DM_{cosmic} =
220~pc~cm^{-3}$), 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 with
foreground halos at small impact parameters ($\lesssim 100$ kpc). To test this
hypothesis, we carried out spectroscopic observations of the field galaxies
within $\sim$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, $\rm DM_{halos}< 45~pc~cm^{-3}$ (95 % c.l.). This
value 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 Aemulus
halo mass function and reasonably explains its low $\rm DM_{cosmic}$ 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.
