Very Extended X‐Ray and Hα Emission in M82: Implications for the Superwind Phenomenon

We discuss the properties and implications of a 3.7x0.9 kpc region ofspatially-coincident X-ray and H-alpha emission about 11.6 kpc to the north ofthe galaxy M82 previously discussed by Devine and Bally (1999). The PSPC X-rayspectrum is fit by thermal plasma (kT=0.80+-0.17 keV) absorbed by only theGalactic foreground column density. We evaluate the relationship of theX-ray/H-alpha ridge to the M82 superwind. The main properties of the X-rayemission can all be explained as being due to shock-heating driven as thesuperwind encounters a massive ionized cloud in the halo of M82. This encounterdrives a slow shock into the cloud, which contributes to the excitation of theobserved H-alpha emission. At the same time, a fast bow-shock develops in thesuperwind just upstream of the cloud, and this produces the observed X-rayemission. This interpretation would imply that the superwind has an outflowspeed of roughly 800 km/s, consistent with indirect estimates based on itsgeneral X-ray properties and the kinematics of the inner kpc-scale region ofH-alpha filaments. The gas in the M82 ridge is roughly two orders-of-magnitudehotter than the minimum "escape temperature" at this radius, so this gas willnot be retained by M82. (abridged)Comment: 24 pages (latex), 3 figures (2 gif files and one postscript), accepted for publication in Part 1 of The Astrophysical Journa