Hα Emission 11 Kiloparsecs above M82

Abstract

We report the discovery of Ha 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 identiÐed emission-line components associated with the nuclear superwind. The Ha emission can be traced nearly continuously for 10@ northwest of M82 out to an emission-line structure (the "" cap ÏÏ) that lies at a projected distance of 11È12 kpc from the M82 nucleus. The cap has a shell-like morphology , is blueshifted by 50È200 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 Ha 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.