Kavli Affiliate: Kiyoshi Masui
| Summary:
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 $r_rm vir= 302,mathrmkpc$) to a control sample of 684 FRBs, we estimate the DM contribution from M31’s CGM. We find evidence for an excess DM of $δmathrmDM = 5.9$–$59.6,mathrmpc,cm^-3$ in the inner halo ($approx 0$–$151,mathrmkpc$) and $δmathrmDM = 25.7$–$64.6,mathrmpc,cm^-3$ in the outer halo ($approx 151$–$302,mathrmkpc$). Using a generalized halo model parameterized by a closure radius $r_mathrmclose$, we constrain the baryon distribution and infer a best-fit value for $r_mathrmclose$ to be $9.2^+9.9_-4.9,r_rm vir$ ($1σ$), corresponding to a total CGM mass of $M_b,mathrmhalo = 18.6^+7.9_-8.4 times 10^10,M_odot$. 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.
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