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HST Imaging of Dust Structures and Stars in the Ram Pressure Stripped Virgo Spirals NGC 4402 and NGC 4522: Stripped from the Outside In with Dense Cloud Decoupling

We describe and constrain the origins of ISM structures likely created by ongoing ICM ram pressure stripping in two Virgo Cluster spirals, NGC 4522 and NGC 4402, using HST BVI images of dust extinction and stars, as well as supplementary HI, Halpha, and radio continuum images. This is the highest-resolution study to date of the physical processes that occur during an ICM-ISM ram pressure stripping interaction, ram pressure stripping's effects on the multi-phase, multi-density ISM, and the formation and evolution of ram-pressure-stripped tails. In dust extinction, we view the leading side of NGC 4402 and the trailing side of NGC 4522; we see distinct types of features in both galaxies. NGC 4522 has experienced stronger, more recent pressure and has the jellyfish morphology characteristic of some ram pressure stripped galaxies. Its stripped tail extends up from the disk plane in continuous upturns of dust and stars curving ~2 kpc above the disk plane. A kinematically and morphologically distinct extraplanar arm of young, blue stars and ISM extends above a mostly-stripped portion of the disk, and between it and the disk plane are decoupled dust clouds. NGC 4402 contains long dust ridges, suggesting that large parts of the ISM are being pushed out at once. Both galaxies contain long ridges of polarized radio continuum emission indicating the presence of large-scale ordered magnetic fields. We propose that magnetic fields could bind together gas of different densities, causing nearby gas of different densities to be stripped at the same rate and creating the large, coherent dust ridges and upturns. A number of factors that play roles in determining what types of structures form as a result of ram pressure: ram pressure strength and history, the location within the galaxy relative to the leading side, and pre-existing substructure in the ISM that may be bound together by magnetic fields.