  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. 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. 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. The galaxy is 3.3 degrees ~800 kpc from
M87, somewhat outside the region of strongest cluster X-ray emission. The ram
pressure may be 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, implying that the star-forming molecular ISM has
been effectively stripped from the outer disk of the galaxy along with the HI.

Astrophysics