VLA H I Observations of Gas Stripping in the Virgo Cluster Spiral NGC 4522

We present VLA H I observations at ∼20'' ≃ 1.5 kpc resolution of the highly inclined, H I–deficient Virgo Cluster spiral galaxy NGC 4522, which is one of the clearest and nearest cases of ongoing intracluster medium–interstellar medium (ICM-ISM) stripping. H I is abundant and spatially coincident with the stellar disk in the center, but beyond R = 3 kpc the H I distribution in the disk is sharply truncated, and the only H I is extraplanar and all on the northwest side. Forty percent of the total H I, corresponding to 1.5 × 10⁸ M_⊙, is extraplanar and has likely been removed from the galaxy disk by an ICM-ISM interaction. The kinematics and the morphology of the H I appear more consistent with ongoing stripping and less consistent with gas fall-back, which may occur long after peak pressure. Some of the extraplanar gas has line widths (FWZI) of 150 km s^-1, including a blueshifted tail of weaker emission, and much of the extraplanar gas exhibits a modest net blueshift with respect to the galaxy's disk rotational velocities, consistent with gas accelerated toward the mean cluster velocity. The southwest side of the galaxy has less H I in the disk but more H I in the halo, suggesting more effective gas removal on the side of the galaxy that is rotating into the ICM wind. In recent simulations of ICM-ISM interactions large surface densities of extraplanar gas like that in NGC 4522 are seen at relatively early stages of active stripping and not during later gas fall-back stages. The galaxy is 33 ≃ 800 kpc from M87, somewhat outside the region of strongest cluster X-ray emission. The ram pressure at this location, assuming a static smooth ICM and standard values for ICM density and galaxy velocity, appears inadequate to cause the observed stripping. We consider the possibility that the ram pressure is significantly stronger than standard values, because of large bulk motions and local density enhancements of the ICM gas, which may occur in a dynamic, shock-filled ICM experiencing subcluster merging. The H I and Hα distributions are similar, with both truncated in the disk at the same radius and H II regions located throughout much of the extraplanar H I. This implies that the star-forming molecular ISM has been effectively stripped from the outer disk of the galaxy along with the H I. The inferred peak stripping rate of ∼10 M_⊙ yr^-1 is much larger than the galaxy's total star formation rate of ∼0.1 M_⊙ yr^-1, implying that the rate of triggered star formation due to ICM pressure is presently minor compared with the rate of gas lost as a result of stripping.