Hα Emission 11 Kiloparsecs above M82

We report the discovery of Hα emission associated with the redshifted lobe of the M82 superwind extending out to a projected distance of 11 kpc from the disk of M82, which is 3 times farther than previously identified emission-line components associated with the nuclear superwind. The Hα emission can be traced nearly continuously for 10' northwest of M82 out to an emission-line structure (the "cap") that lies at a projected distance of 11-12 kpc from the M82 nucleus. The cap has a shell-like morphology, is blueshifted by 50-200 km s -1 relative to the M82 nucleus, and is visible in a ROSAT PSPC image of the region. We consider two possible models for the Hα bright cap: it may be a bow shock formed by the impact of the superwind, either with previously emitted wind material or with ambient intergalactic material and/or tidal debris left over from the interaction between M81 and M82; or it may trace photoionized material illuminated by Lyman continuum photons leaking out of the M82 nuclear region through the hot bipolar cavity produced by the starburst-driven superwind.