Ongoing Gas Stripping in the Virgo Cluster Spiral NGC 4522
  The Virgo cluster galaxy NGC 4522 is one of the best spiral candidates for ICM-ISM stripping in action. Optical broadband and H-alpha images from the WIYN telescope of the highly inclined galaxy reveal a relatively undisturbed stellar disk and a peculiar distribution of H-alpha emission. Ten percent of the H-alpha emission arises from extraplanar HII regions which appear to lie within filamentary structures >3 kpc long above one side of the disk. The filaments emerge from the outer edge of a disk of bright H-alpha emission which is abruptly truncated beyond 0.35R(25). Together the truncated H-alpha disk and extraplanar H-alpha filaments are reminiscent of a bow shock morphology, which strongly suggests that the interstellar medium (ISM) of NGC 4522 is being stripped by the gas pressure of the intracluster medium (ICM). The galaxy has a line-of-sight velocity of 1300 km/sec with respect to the mean Virgo cluster velocity, and thus is expected to experience a strong interaction with the intracluster gas. The existence of HII regions apparently located above the disk plane suggests that star formation is occuring in the stripped gas, and that newly formed stars will enter the galaxy halo and/or intracluster space. The absence of HII regions in the disk beyond 0.35R(25), and the existence of HII regions in the stripped gas suggest that even molecular gas has been effectively removed from the disk of the galaxy. 
Jeffrey D. P. Kenney and Rebecca A. Koopmann