  Deep wide-field H$\alpha$+[NII] imaging around the Virgo cluster giant
elliptical galaxy M86 reveals a highly complex and disturbed ISM/ICM. The most
striking feature is a set of H$\alpha$ filaments which clearly connect M86 with
the nearby disturbed spiral NGC 4438 (23$'$=120 kpc projected away), providing
strong evidence for a previously unrecognized collision between them.
Spectroscopy of selected regions show a fairly smooth velocity gradient between
M86 and NGC 4438, consistent with the collision scenario. Such a collision
would impart significant energy into the ISM of M86, probably heating the gas
and acting to prevent the gas from cooling to form stars. We propose that cool
gas stripped from NGC 4438 during the collision and deposited in its wake is
heated by shocks, ram pressure drag, or thermal conduction, producing most of
the H$\alpha$ filaments. Some H$\alpha$ filaments are associated with the
well-known ridge of bright X-ray emission to the NW of the nucleus, suggesting
that the collision is responsible for peculiarities of M86 previously ascribed
to other effects. M86 is radio-quiet, thus AGN heating is unlikely to play a
significant role. The M86 system has implications for understanding the role of
gravitational interactions in the heating of the ISM in ellipticals, and how
collisions in clusters transform galaxies.
