2021ApJ...907...14Q

We report a large-scale (r~20^○) X-ray-bright and Sunyaev-Zeldovich (SZ)-bright diffuse enhancement toward M31, which might be a Local Hot Bridge connecting the Milky Way (MW) with M31. We subtract the Galactic emission from the all-sky O VII and O VIII emission-line measurement survey, and find that the emission of these two ions is enhanced within r~20^○ projected around M31. The mean emission enhancements are 5.6 ± 1.3 L.U., and 2.8 ± 0.6 L.U. for O VII and O VIII, respectively (> 4σ for both ions). We also extract the SZ signal around M31, which suggests a surface brightness y of 2-4×10^–7, an enhancement > 2.5σ (and a best fit of 5.9σ ). These three measurements trace the hot gas with a temperature logT(K)> 6, showing similar plateau shapes (flat within ~15^○, and zero beyond ≃30°). A single-phase assumption leads to a temperature of logT(K)=6.34±0.03, which is determined by the O VII/O VIII line ratio. Combining X-ray and SZ measurements, we suggest that this feature is unlikely to be the hot halo around M31 (too massive) or in the MW (too high pressure and X-ray bright). The plateau shape may be explained by a cylinder connecting the MW and M31 (the Local Hot Bridge). We constrain its length to be about 400 kpc, with a radius of 120 kpc, a density of ~2×10^–4–10^–3cm^–3, and a metallicity of 0.02–0.1Z_☉. The baryon mass is ≳10¹¹M_☉, and the oxygen mass is about ≳10⁸M_☉, which contribute to the baryon or metal budget of the Local Group.