HI as a Probe of Structure in the Interstellar Medium of External Galaxies

This review presents a perspective on recent advances in understanding neutral ISM structure in external galaxies. HI is a fundamental probe of galactic baryonic material, and its structure and distribution offer vital signatures of dynamical and evolutionary processes that drive star formation and galaxy evolution. New, high-resolution HI data cubes for external galaxies now reveal the features and topology of the entire neutral ISM, which here are considered on scales of 10 - 1000 pc. I focus on the two principal candidates for HI structuring, mechanical feedback from massive stars and turbulence; other mechanisms are also considered, especially with respect to supergiant shells. While confirmation for both mechanical feedback and turbulent processes exists, it remains unclear how these mechanisms yield the global, steady-state, scale-free HI properties that are observed. Understanding the formation of filamentary structure may be key in resolving these puzzles. New HI surveys of nearby galaxies, combined with further theoretical studies, promise continuing important advances