HST and HI Imaging of Strong Ram Pressure Stripping in the Coma Spiral
  NGC 4921: Dense Cloud Decoupling and Evidence for Magnetic Binding in the ISM
  Remarkable dust extinction features in the deep HST V and I images of the
face-on Coma cluster spiral galaxy NGC 4921 show in unprecedented ways how ram
pressure strips the ISM from the disk of a spiral galaxy. New VLA HI maps show
a truncated and highly asymmetric HI disk with a compressed HI distribution in
the NW, providing evidence for ram pressure acting from the NW. Where the HI
distribution is truncated in the NW region, HST images show a well-defined,
continuous front of dust that extends over 90 degrees and 20 kpc. This dust
front separates the dusty from dust-free regions of the galaxy, and we
interpret it as galaxy ISM swept up near the leading side of the ICM-ISM
interaction. We identify and characterize 100 pc-1 kpc scale substructure
within this dust front caused by ram pressure, including head-tail filaments,
C-shaped filaments, and long smooth dust fronts. The morphology of these
features strongly suggests that dense gas clouds partially decouple from
surrounding lower density gas during stripping, but decoupling is inhibited,
possibly by magnetic fields which link and bind distant parts of the ISM.
