Tentative Excess Dispersion Measures toward DESI Galaxy Groups from Localized Fast Radio Bursts

Fast radio bursts (FRBs) provide sensitive probes of ionized baryons through their dispersion measures (DMs). In this work, we test whether localized FRB sight lines intersecting foreground galaxy groups exhibit a measurable excess in cosmic DM relative to clean nonintersecting sight lines. We crossmatch 117 localized FRBs with measured DMs and host redshifts against a foreground group catalog from the DESI Legacy Imaging Surveys. Within the catalog footprint and at z host < 0.5, we identify 23 sight lines intersecting foreground groups and 18 clean nonintersecting sight lines for the main comparison. A joint fit to this sample gives a positive offset of 143 . 3 − 53.0 + 53.2 pc cm − 3 in the inferred cosmic DM, DM cosmic , for the group-intersecting sight lines relative to the nonintersecting control sample. A template fit for the foreground halos gives A halo = 0.3 4 − 0.50 + 0.51 , with a positive posterior median but a weakly constrained detailed halo contribution. A mock forecast within the DESI footprint indicates that larger samples of localized FRBs would improve the foreground halo-template constraint.