Constraining the mass of the M31 ionized baryon Halo using CHIME/FRB Catalog 2

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 $r_{\mathrm{v}\mathrm{i}\mathrm{r}}=302\mathrm{k}\mathrm{p}\mathrm{c}$) to a control sample of 684 FRBs, we estimate the DM contribution from M31’s CGM. We find evidence for an excess DM of $\delta \mathrm{D}\mathrm{M}=5.9$–$59.6\mathrm{p}\mathrm{c}\mathrm{c}{\mathrm{m}}^{-3}$ in the inner halo ($\approx 0$–$151\mathrm{k}\mathrm{p}\mathrm{c}$) and $\delta \mathrm{D}\mathrm{M}=25.7$–$64.6\mathrm{p}\mathrm{c}\mathrm{c}{\mathrm{m}}^{-3}$ in the outer halo ($\approx 151$–$302\mathrm{k}\mathrm{p}\mathrm{c}$). Using a generalized halo model parameterized by a closure radius $r_{\mathrm{c}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{e}}$, we constrain the baryon distribution and infer a best-fit value for $r_{\mathrm{c}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{e}}$ to be $9.2_{-4.9}^{+9.9}{r}_{\mathrm{v}\mathrm{i}\mathrm{r}}$ ($1\sigma$), corresponding to a total CGM mass of $M_{b,\mathrm{h}\mathrm{a}\mathrm{l}\mathrm{o}}=18.6_{-8.4}^{+7.9}\times {10}^{10}{M}_{\odot }$. 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.