HI Observations of Gas Stripping in the Virgo Cluster Spiral NGC 4522

We present VLA HI observations at ∼20′′≃1.5 kpc resolution of the highly inclined, HI-deficient Virgo cluster spiral galaxy NGC 4522, which is one of the clearest and nearest cases of ongoing ICM-ISM stripping. HI is abundant and spatially coincident with the stellar disk in the center, but beyond R = 3 kpc the HI distribution in the disk is sharply truncated and the only HI is extraplanar, and all on the northwest side. Forty percent of the total HI, 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 HI 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 linewidths (FWZI) of 150 km s, 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 SW side of the galaxy has less HI in the disk but more HI in the halo, suggesting more effective gas removal on the side of the galaxy which 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 fallback stages. The galaxy is 3.3◦≃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 Yale University Astronomy Department, P.O. Box 208101, New Haven, CT 06520-8101 USA Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027 CDS, Observatoire Astronomique de Strasbourg, UMR 7550, 11 Rue de l’Universite, 67000 Strasbourg, France