The Small-scale Structures in the Wind of Messier 82

Small-scale multiphase structure plays a central role in galactic-wind evolution, yet the parsec-scale morphology and excitation of the warm ionised gas remain poorly constrained. We present deep HST narrow-band imaging of the southern wind of Messier 82 (M82) in H$\alpha$, [OIII], [SII], and [NII], designed to resolve the warm ionised phase on parsec scales. The H$\alpha$ emission is detected to $\approx$2.1 kpc above the disk, while the fainter emission lines are detected over smaller radial extents, with [OIII] reaching $\approx$ 1.5 kpc. We develop a filament-finding pipeline for the H$\alpha$ image and construct a quantitative catalogue of the filamentary structures. The wind forms a highly connected network of strands and knots, dominated by compact filaments with typical projected widths of $\approx$5.3 pc and lengths of $\approx$ 9.5 pc. Both the projected covering fraction and the line-flux contribution of the filamentary component decline with height, showing that the outer wind becomes increasingly dominated by diffuse emission. Optical line-ratio diagnostics indicate that the warm ionised gas occupies an intermediate excitation regime: photoionisation by the central starburst can energetically power the observed H$\alpha$ luminosity, while the systematic separation between filamentary and diffuse emission, together with the evolution of the line ratios with vertical distance, suggests an increasing contribution from shocks or other similar heating in the diffuse outer wind. These results show that separating filamentary and diffuse emission in high-resolution imaging provides a powerful way to connect the morphology, excitation, and multiphase structure of galactic winds.