PIFFLE: Characterizing the Foreground Contributions from 4 Decades in Halo Mass to the FRB20230907D Dispersion Measure

We characterize the foreground environment of FRB20230907D, localized to a galaxy at z=0.464, which has an observed dispersion measure of DM obs=1031 pc cm-3. At its redshift, FRB20230907D lies above the Macquart relation, the expected relation between cosmological dispersion measure and the source redshift, indicating a substantial excess DM along this line of sight. We use Subaru/PFS and SDSS spectroscopy, published group catalogs, Rubin/LSST imaging, and eROSITA X-ray data to characterize the foreground structures that may account for this excess. A friends-of-friends search identifies a massive foreground system at z 0.09 with M 200 5.2 times10 14 M, while low redshift catalogs reveal an additional group at z 0.02565. Assuming that the halo gas follows a modified-NFW halo density profile, we estimate observer frame contributions of 150+110-70 pc cm-3 and 80+60-40 pc cm-3 from these systems, respectively. Together with the Milky Way, diffuse intergalactic medium, Virgo cluster, M49 group, and host galaxy contributions, these foreground structures can account for the excess dispersion measure of FRB20230907D within uncertainties. This highlights the importance of dense foreground spectroscopy and multi-wavelength data.