The circumgalactic medium (CGM) surrounding galaxies is believed to be a significant reservoir of baryons, yet its total mass remains poorly constrained. We present a novel approach to probe the CGM of the Andromeda galaxy (M31) using fast radio bursts (FRBs) from the CHIME/FRB Catalog 2. By comparing the dispersion measures (DMs) of 171 FRBs whose sightlines intersect M31's halo (within $r_{ \rm vir}= 302\,\mathrm{ kpc}$) to a control sample of 684 FRBs, we estimate the DM contribution from M31's CGM. We find evidence for an excess DM of $\delta\mathrm{ DM} = 5.9$--$59.6\,\mathrm{ pc\,cm^{ -3}}$ in the inner halo ($\approx 0$--$151\,\mathrm{ kpc}$) and $\delta\mathrm{ DM} = 25.7$--$64.6\,\mathrm{ pc\,cm^{ -3}}$ in the outer halo ($\approx 151$--$302\,\mathrm{ kpc}$). Using a generalized halo model parameterized by a closure radius $r_{ \mathrm{ close}}$, we constrain the baryon distribution and infer a best-fit value for $r_{ \mathrm{ close}}$ to be $9.2^{ +9.9}_{ -4.9}\,r_{ \rm vir}$ ($1\sigma$), corresponding to a total CGM mass of $M_{ b,\mathrm{ halo}} = 18.6^{ +7.9}_{ -8.4} \times 10^{ 10}\,M_\odot$. Our results suggest that M31 may harbor a substantial fraction of its cosmic baryon budget in diffuse, ionized gas. This work demonstrates the potential of FRBs as a powerful tool for studying the CGM of nearby galaxies, with future larger samples expected to provide tighter constraints on the baryon content of galactic halos.
Categories: astro-ph.GA
