2025ApJ...989..100L

Star formation feedback can drive large-scale, multiphase galactic outflows. The dynamical and thermodynamical interaction between the hot and cooler phases is a prime focus of both observational and theoretical work. Here, we analyze Hα-emitting structures in the extraplanar wind of the nearby starburst M82. We use high-resolution, narrowband, observations from the Hubble Legacy Archive. Our analysis constrains the morphology, number density, and column density of the structures. We highlight conspicuous arc-like structures that differ significantly from the linear cometary clouds that emerge from galactic wind simulations and discuss their possible origins, such as bow shocks or instabilities driven by cosmic rays. The most prominent structures range in size from ∼24 to 110 pc. Using the Hα brightness and assumptions about the depth of the emitting structures, we estimate number densities of ∼1-23 cm^–3 assuming a unity volume filling factor, which are lower than previous constraints from spectroscopic nebular line studies. The derived column densities, ∼10²⁰-10²¹ cm^–2, along the path of the outflow are above theoretical thresholds for cool cloud survival in a hot supersonic background, but small enough that the structures could be accelerated by the hot wind momentum. Using diffuse X-ray emission maps from Chandra, we also find that even on small (∼100 pc) scales, the Hα “leads” the X-rays, a behavior long noted in the literature on kiloparsec scales. This behavior, along with previous observational studies of ionization in the wind, may signal that shock ionization is responsible for the Hα emission we observe.