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

Abstract

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 $δ\mathrm{DM} = 5.9$--$59.6\,\mathrm{pc\,cm^{-3}}$ in the inner halo ($\approx 0$--$151\,\mathrm{kpc}$) and $δ\mathrm{DM} = 25.7$--$64.6\,\mathrm{pc\,cm^{-3}}$ 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 $9.2^{+9.9}_{-4.9}\,r_{\rm vir}$ ($1σ$), 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.

Figures (1)

• Figure 1: Skymap of the final analysis catalog (built from FRBs in CHIME/FRB Catalog 2 collaboration_second_2026) showing the locations of $171$ FRBs intersecting M31's halo (within $r_{\mathrm{vir}}$) and the corresponding control sample of $684$ FRBs. The ellipse markers represent the $\approx3$ localisations of each FRB. The inset in the bottom left region shows the extent of the M31 halo and the predicted $\mathrm{DM_{M31,halo}}$ contribution from a generalized halo model (see Section \ref{['sec:cgm_profile']}) with an assumed $r_{\mathrm{close}} = 10.0~r_{\rm vir}$. The overdensity of detected FRBs at the Celestial North Pole is due to the orientation of CHIME, which is positioned to face North-South and is located in the Northern Hemisphere; therefore, it is always oriented North-South and the North Pole is always in the field of view.