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

TL;DR

This study examines the morphology, density, and column density of cool gas clouds in M82's starburst-driven outflow, revealing structures that challenge existing models and suggest shock ionization as a key process.

Contribution

It provides high-resolution observational constraints on cool gas structures in M82's outflow, highlighting their morphology, densities, and possible origins, which differ from previous simulation predictions.

Findings

Arc-like structures range from 24 to 110 pc in size.

Estimated densities are 1-23 cm$^{-3}$, lower than previous spectroscopic constraints.

H$ extalpha$ emission leads X-ray emission, indicating shock ionization.

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 $\sim24 -110$ pc. Using the H$\alpha$ brightness and assumptions about the depth of the emitting structures, we estimate number densities of $\sim1-23$ cm$^{-3}$ assuming a unity volume filling factor, which are lower than previous constraints from spectroscopic nebular line studies. The derived column densities, $\sim10^{20}-10^{21}$ 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 $\textit{Chandra}$, we also find that even on small ($\sim100$ 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.