Ongoing Gas Stripping in the Virgo Cluster Spiral Galaxy NGC 4522

The Virgo Cluster galaxy NGC 4522 is one of the best spiral candidates for intracluster medium–interstellar medium (ICM-ISM) stripping in action. Optical broadband and Hα images from the WIYN telescope of the highly inclined galaxy reveal a relatively undisturbed stellar disk and a peculiar distribution of Hα emission. Ten percent of the Hα emission arises from extraplanar H II 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 Hα emission which is abruptly truncated beyond 0.35R₂₅. Together the truncated Hα disk and extraplanar Hα filaments are reminiscent of a bow shock morphology, which strongly suggests that ISM of NGC 4522 is being stripped by the gas pressure of the ICM. The galaxy has a line-of-sight velocity of ≃1300 km s^-1 with respect to the mean Virgo Cluster velocity and thus is expected to experience a strong interaction with the intracluster gas. The existence of H II regions apparently located above the disk plane suggests that star formation is occurring in the stripped gas, and that newly formed stars will enter the galaxy halo and/or intracluster space. The absence of H II regions in the disk beyond 0.35R₂₅ and the existence of H II regions in the stripped gas suggest that even molecular gas has been effectively removed from the disk of the galaxy.