Search for a Dispersion Measure Excess Toward Nearby Galaxy Halos Using Fast Radio Bursts from the Second CHIME/FRB Baseband Catalog

Fast radio bursts (FRBs) probe foreground ionized baryons via their dispersion measures (DM). Using 1748 sub-arcminute-localized sources from the Second CHIME/FRB Baseband Catalog, we conduct a statistical stacking study of the circumgalactic media (CGM) of nearby galaxies by analyzing the DMs of FRBs whose sightlines intersect foreground halos. For galaxy halos within 50 Mpc, we measure a mean DM excess of 119±44 pc~cm⁻³ (p = 0.0053; ~ 2.6σ) within one virial radius. Expanding to 150 Mpc (628 intersections), we find a tentative decline in the mean DM as a function of impact parameter (p = 0.0046; ~ 2.6σ), which is predominantly driven by typical Milky Way (MW)-like galaxies (10 ≤ log(M_*/M₎≤ 11). We use this DM profile to constrain parameters of the CGM electron density distribution, nₑCGM(r) ∝ r-3β, finding an extrapolated central electron density in the range of (1.7-21.1) × 10⁻³\,cm⁻³, with a radial power-law index of β= 0.68+0.15-0.16, consistent with constraints from X-ray stacking studies of MW-like galaxies. At very low impact parameters, we detect a significant DM deficit likely due to cross-matching FRBs that originate from galaxies within our galaxy sample. By deriving the DM-galaxy cross-correlation function, we show that this deficit matches theoretical predictions if 20 FRBs originate from galaxies within 150 Mpc, indicating that CHIME/FRB detects a higher fraction of low-z FRBs than previously thought. We forecast that 10⁴ FRBs with similar localization errors will provide a 5-$σ$ detection of the CGM, and that FRBs at intermediate redshifts (0.4 zFRB 0.75) are ideal for studying intervening CGMs.