We discuss the properties and implications of the 3.5 \times 0.9 arc minute ( 3.7 \times 0.9 kpc ) region of spatially-coincident X-ray and H \alpha emission about 11 arc minutes ( 11.6 kpc ) to the north of the prototypical starburst/superwind galaxy M 82 previously discussed by [ Devine & Bally 1999 ] .
The total H \alpha flux from this ridge of emission is 1.5 \times 10 ^ { -13 } ergs s ^ { -1 } cm ^ { -2 } , or about 0.3 % of the total M 82 H \alpha flux .
The implied H \alpha luminosity of this region is 2.4 \times 10 ^ { 38 } ergs s ^ { -1 } .
Diffuse soft X-ray emission is seen over the same region by the ROSAT PSPC and HRI .
The PSPC X-ray spectrum is fit by thermal plasma absorbed by only the Galactic foreground column density ( N _ { H } = 3.7 \times 10 ^ { 20 } cm ^ { -2 } ) and having a temperature of kT=0.80 \pm 0.17 keV .
The total unabsorbed flux from the ridge is 1.4 \times 10 ^ { -13 } ergs cm ^ { -2 } s ^ { -1 } ( \sim 2.2 \times 10 ^ { 38 } ergs s ^ { -1 } ) , comprising about 0.7 % of the total X-ray emission from M 82 .

We evaluate the relationship of the X-ray/H \alpha ridge to the M 82 superwind .
The H \alpha emission could be excited by ionizing radiation from the starburst that propagates into the galactic halo along the cavity carved by the superwind .
However , the main properties of the X-ray emission can all be explained as being due to shock-heating driven as the superwind encounters a massive ionized cloud in the halo of M 82 ( possibly related to the tidal debris seen in HI in the interacting M 81/M 82/NGC 3077 system ) .
This encounter drives a slow shock into the cloud , which contributes to the excitation of the observed H \alpha emission .
At the same time , a fast bow-shock develops in the superwind just upstream of the cloud , and this produces the observed X-ray emission .
This interpretation would imply that the superwind has an outflow speed of roughly 800 km s ^ { -1 } , consistent with indirect estimates based on its general X-ray properties and the kinematics of the inner kpc-scale region of H \alpha filaments .
The alternative , in which a much faster and more tenuous wind drives a fast radiative shock into a cloud that then produces both the X-ray and H \alpha emission is ruled out by the long radiative cooling times and the relatively quiescent H \alpha kinematics in this region .

We suggest that wind-cloud interactions may be an important mechanism for generating X-ray and optical line emission in the halos of starbursts .
Such interactions can establish that the wind has propagated out to substantially greater radii than could otherwise be surmised .
This has potentially interesting implications for the fate of the outflowing metal enriched material , and bears on the role of superwinds in the metal enrichment and heating of galactic halos and the intergalactic medium .
In particular , the gas in the M 82 ridge is roughly two orders-of-magnitude hotter than the minimum “ escape temperature ” at this radius , so this gas will not be retained by M 82 .