Evidence for a Hot Galactic Halo around the Andromeda Galaxy Using Fast Radio Bursts along Two Sightlines

Abstract Fast radio bursts (FRBs) are millisecond-duration radio transients that serve as unique probes of ionizedextragalactic 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 pc cm −3 toward FRB 20230930A and 51–366 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 pc cm −3 along FRB 20230930A (90% CI). The inferred values of DM M31,halo from the FRBs are consistent with predictions from a modified Navarro–Frenk–White profile at the corresponding impact parameter. The cool and warm phase gas is unlikely to account for the 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.

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Publication Details

Journal
The Astrophysical Journal
Published
2025-11-06
DOI
https://doi.org/10.3847/1538-4357/ae1014
Citations
2
Primary Topic
Pulsars and Gravitational Waves Research
Type
article
Field-Weighted Citation Impact
2.31
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article

Evidence for a Hot Galactic Halo around the Andromeda Galaxy Using Fast Radio Bursts along Two Sightlines

Fabian Walter, Geoffrey C. Bower, Kritti Sharma, Wen‐fai Fong et al.
2 citations
The Astrophysical Journal
Pulsars and Gravitational Waves Research
2.31
article

Evidence for a Hot Galactic Halo around the Andromeda Galaxy Using Fast Radio Bursts along Two Sightlines

Fabian Walter, Geoffrey C. Bower, Kritti Sharma, Wen‐fai Fong, Benjamin F. Williams, Jason Xavier Prochaska, Sarah Burke-Spolaor, Nickolas M. Pingel, Liam Connor, James Stanley, Anya E. Nugent, Paul B. Demorest, Casey Law, Laura B. Chomiuk, Reshma Anna-Thomas, Alexa C. Gordon, Eric W. Koch, Zhuo Chen, Calvin Dear, Frank Verdi
article en
2 citations

Abstract

Abstract Fast radio bursts (FRBs) are millisecond-duration radio transients that serve as unique probes of ionizedextragalactic 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 pc cm −3 toward FRB 20230930A and 51–366 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 pc cm −3 along FRB 20230930A (90% CI). The inferred values of DM M31,halo from the FRBs are consistent with predictions from a modified Navarro–Frenk–White profile at the corresponding impact parameter. The cool and warm phase gas is unlikely to account for the 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.

The Astrophysical JournalVol. 993(2)
Northwestern University (US), University of Hawaiʻi at Mānoa (US), West Virginia University (US), California Institute of Technology (US), University of Wisconsin System (US), University of Wisconsin–Madison (US), Kavli Institute for the Physics and Mathematics of the Universe (JP), University of California, Santa Cruz (US), University of Washington (US), National Radio Astronomy Observatory (US), Max Planck Institute for Astronomy (DE), Center for Astrophysics Harvard & Smithsonian (US), National Astronomical Observatory of Japan (JP), Michigan State University (US), University of Amsterdam (NL), Netherlands Institute for Radio Astronomy (NL)
Openalex Percentile: Top 10%
Pulsars and Gravitational Waves Research
2.31
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