We present VLA HI observations at \sim 20 ^ { \prime \prime } \simeq 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 \times 10 ^ { 8 } M _ { \sun } , 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 ^ { -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 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 fall-back stages .
The galaxy is 3.3 ^ { \circ } \simeq 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 , due to large bulk motions and local density enhancements of the ICM gas , which may occur in a dynamic , shock-filled ICM experiencing subcluster merging .
The HI and H \alpha distributions are similar , with both truncated in the disk at the same radius and HII regions located throughout much of the extraplanar HI .
This implies that the star-forming molecular ISM has been effectively stripped from the outer disk of the galaxy along with the HI .
The inferred peak stripping rate of \sim 10 M _ { \sun } yr ^ { -1 } 
is much larger than the galaxy ’ s total star formation rate of \sim 0.1 M _ { \sun } yr ^ { -1 } , implying that the rate of triggered star formation due to ICM pressure is presently minor compared to the rate of gas lost due to stripping .