Observational Constraints on Cool Gas Clouds in M82's Starburst-Driven Outflow

Abstract

Star formation feedback can drive large-scale, multi-phase 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 H$\alpha$-emitting structures in the extraplanar wind of the nearby starburst M82. We use high-resolution, narrow-band, observations from the Hubble Legacy Archive (Mutchler et al. 2007). 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 $\sim 24-110$ pc. Using the H$\alpha$ brightness and assumptions about the depth of the emitting structures, we estimate number densities of $\sim 1-23$ cm${}^{-3}$ assuming a unity volume filling factor, which are lower than previous constraints from spectroscopic nebular line studies. The derived column densities, $\sim 10^{20}-10^{21}$ 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 $\text{Chandra}$, we also find that even on small ($\sim 100$ pc) scales, the H$\alpha$ "leads" the X-rays, a behavior long noted in the literature on kiloparsec scales, and one we observe in the brightness profiles of the structures we analyze. This behavior, along with previous observational studies of ionization in the wind, may signal that shock ionization is responsible for the H$\alpha$ emission we observe.

22 pages, 9 Figures; submitted to ApJ (10 February 2025); Accepted by ApJ (3 July 2025)

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• JWST Observations of Starbursts: Dust Processing in the M82 Superwind
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• Modeling Emission-Line Surface Brightness in a Multiphase Galactic Wind: An O VI Case Study