An X-ray and SZ bright diffuse source toward M31: a Local Hot Bridge
  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 $r\approx20^\circ$ around
M31. The mean emission enhancements are $5.6\pm 1.3$ L.U., and $2.8\pm0.6$ 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
$2-4\times10^{-7}$, an enhancement $>2.5\sigma$ (and a best fit of
$5.9\sigma$). These three measurements trace the hot gas with a temperature
$\log~T({\rm K})> 6$, showing similar plateau shapes (flat within
$\approx15^\circ$, and zero beyond $\approx30^\circ$). A single-phase
assumption leads to a temperature of $\log~T({\rm K})=6.34\pm0.03$, 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 $\approx 2\times10^{-4}-10^{-3} ~\rm cm^{-3}$, and a metallicity
of $0.02-0.1~ Z_\odot$. The baryon mass is $\gtrsim10^{11}~M_\odot$, and the
oxygen mass is about $\gtrsim10^8~M_\odot$, which contribute to the baryon or
metal budget of the Local Group.
