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

TL;DR

This study uses HST and VLA HI imaging to reveal how ram pressure strips the interstellar medium in NGC 4921, showing dense cloud decoupling and magnetic binding effects during galaxy-ICM interactions.

Contribution

It provides detailed imaging evidence of ram pressure stripping effects, highlighting the role of magnetic fields in cloud decoupling within the ISM.

Findings

Truncated and asymmetric HI disk indicating ram pressure effects

Dust front acting as a boundary of ISM swept by ram pressure

Evidence of magnetic fields binding gas clouds during stripping

Abstract

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.