An X-ray and SZ bright diffuse source toward M31: a Local Hot Bridge

Abstract

We report a large-scale ($r\approx 20^{\circ }$) X-ray 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 $r\approx 20^{\circ }$ around M31. The mean emission enhancements are $5.6\pm 1.3$ L.U., and $2.8\pm 0.6$ L.U. for O VII and O VIII, respectively ($>4\sigma$ for both ions). We also extract the SZ signal around M31, which suggests a surface brightness $y$ of $2-4\times 10^{-7}$, an enhancement $>2.5\sigma$ (and a best fit of $5.9\sigma$). These three measurements trace the hot gas with a temperature $\log T(\mathrm{K})>6$, showing similar plateau shapes (flat within $\approx 15^{\circ }$, and zero beyond $\approx 30^{\circ }$). A single-phase assumption leads to a temperature of $\log T(\mathrm{K})=6.34\pm 0.03$, 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 $\approx 2\times 10^{-4}-10^{-3}\mathrm{c}{\mathrm{m}}^{-3}$, and a metallicity of $0.02-0.1Z_{\odot }$. The baryon mass is $\gtrsim 10^{11}{M}_{\odot }$, and the oxygen mass is about $\gtrsim 10^8{M}_{\odot }$, which contribute to the baryon or metal budget of the Local Group.

Submitted to ApJ

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