Evidence for a hot galactic halo around the Andromeda Galaxy using fast radio bursts along two sightlines

TL;DR

This study leverages two FRBs that pierce M31 to probe the electron density of its CGM, separating Milky Way, IGM, and host contributions to constrain the halo’s DM. By modeling the M31 disk and halo along two sightlines, the authors derive DM$_{M31,halo}$ values that are broadly consistent with a hot halo modeled by a modified Navarro-Frenk-White profile. They find that cool and warm CGM components alone cannot explain the observed halo DM, implying a significant hot gas component around M31, though constraints remain limited by only two sightlines and potential foregrounds like a plasma bridge. The work demonstrates FRBs as effective, high-resolution probes of intervening galaxy halos and outlines the path forward with larger FRB samples and refined host-DM priors. Overall, the results provide tentative but encouraging evidence for a hot halo around M31 and illustrate a framework for CGM studies using FRBs.

Abstract

Fast Radio Bursts (FRBs) are millisecond-duration radio transients that serve as unique probes of ionized extragalactic matter. We report the discovery and localization of two FRBs piercing the Andromeda galaxy (M31) with the realfast transient-detection system at the Very Large Array. These unique sightlines enable constraints on M31's electron density distribution. We localized FRB 20230930A to a host galaxy at redshift $z=0.0925$ and FRB 20230506C to a host galaxy at redshift $z=0.3896$. After accounting for the dispersion contributions from the Milky Way, the host galaxies, and the intergalactic medium, we estimate M31's contribution to be $26-239$ $\rm pc~cm^{-3}$ toward FRB 20230930A and $51-366$ $\rm pc~cm^{-3}$ toward FRB 20230506C, within the 90% credible interval (CI). By modeling the M31 disk's contribution, we isolate the halo component and find that M31's halo contributes $7-169$ $\rm pc~cm^{-3}$ along FRB 20230930A (90% CI). The inferred values of $\rm DM_{M31,halo}$ from the FRBs are consistent with predictions from a modified Navarro-Frenk-White (mNFW) profile at the corresponding impact parameter. The cool and warm phase gas is unlikely to account for the $\rm DM_{M31,halo}$ unless the ionization fraction is as high as 90%. While limited to two sightlines, these results offer tentative evidence for the existence of a hot halo surrounding M31. We also discuss the potential contribution of other foreground structures, particularly in explaining the DM excess observed in FRB 20230506C. This work demonstrates how FRBs can be used to probe the circumgalactic medium of intervening galaxies.

Figures (9)

• Figure 1: Dynamic spectrogram (bottom) and frequency‑averaged time profile (top) of the realfast bursts, dedispersed at their detection DM. The data have 10 ms time resolution and 1 MHz frequency resolution; note that channels between 1490 MHz and 1690 MHz were not recorded by the realfast system
• Figure 2: The dirty and clean maps of the realfast localization of the bursts. FRB 20230930A and FRB 20230506C B2 were observed in VLA A-configuration, and the FRB 20230506C B1 was observed in VLA B-configuration.
• Figure 3: (a) The HST/WCS image of the host galaxy of FRB 20230930A . The 5$\sigma$realfast localization region is shown in green. (b) The DBSP spectrum of the host galaxy of FRB 20230930A .
• Figure 4: (a) The Keck R-band image of the host galaxy of FRB 20230506C . The 5$\sigma$realfast localization region is shown in green. (b) The blue region and (c) the red region, of Keck LRIS spectrum of the host galaxy of FRB 20230506C . The redder spectrum had a poor subtraction of telluric features, but we limit our analysis to the features labeled with dashed lines.
• Figure 5: The probability distribution of all different DM components. The left panel corresponds to FRB 20230930A and the right panel to FRB 20230506C . The blue line shows the PDF of the observed DM. The indigo dotted line represents the MW disk contribution, while the dash–dot line represents the MW halo. The green dashed line corresponds to the host DM distribution, the red line to the IGM distribution, and the black line to M31. In the left panel, the dark blue dashed vertical line marks the median of the $\rm DM_{M31}$ distribution, with the shaded region indicating the 90% CI. In the right panel, the orange dashed vertical line marks the median, and the shaded region likewise denotes the 90% CI