An X-Ray- and SZ-bright Diffuse Source toward M31: A Local Hot Bridge

Abstract

Abstract We report a large-scale ( ) 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 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 ( for both ions). We also extract the SZ signal around M31, which suggests a surface brightness y of , an enhancement (and a best fit of ). These three measurements trace the hot gas with a temperature , showing similar plateau shapes (flat within , and zero beyond ≈30°). A single-phase assumption leads to a temperature of , 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 , and a metallicity of . The baryon mass is , and the oxygen mass is about , which contribute to the baryon or metal budget of the Local Group.