The Optical Structure of the Starburst Galaxy M82. II. Nebular Properties of the Disk and Inner-Wind

TL;DR

This study uses optical IFU spectroscopy to analyze the ionized gas properties in M82's starburst region and inner wind, revealing a highly fragmented environment with small cloud sizes and turbulent outflow channels.

Contribution

It provides detailed spatial maps of electron density and ionization states, and characterizes the structure and dynamics of the inner wind and turbulent mixing layers in M82.

Findings

Electron densities peak at a few 1000 cm-3 with small-scale variations.

The inner wind contains a coherent, expanding cylindrical outflow channel.

Turbulent motions increase with height within the starburst energy injection zone.

Abstract

(Abridged) In this second paper of the series, we present the results from optical Gemini-North GMOS-IFU and WIYN DensePak IFU spectroscopic observations of the starburst and inner wind zones of M82, with a focus on the state of the T~10^4 K ionized interstellar medium. Our electron density maps show peaks of a few 1000 cm-3, local small spatial-scale variations, and a fall-off in the minor axis direction. We discuss the implications of these results with regards to the conditions/locations that may favour the escape of individual cluster winds. Our findings imply that the starburst environment is highly fragmented into a range of clouds from small/dense clumps with low filling factors (<1pc, n_e>10^4 cm-3) to larger filling factor, less dense gas. The near-constant state of the ionization state of the ~10^4 K gas throughout the starburst can be explained as a consequence of the small cloud sizes, which allow the gas conditions to respond quickly to any changes. We have examined in more detail both the broad (FWHM 150-350 km/s) line component found in Paper I that we associated with emission from turbulent mixing layers on the gas clouds, and the discrete outflow channel identified within the inner wind. The channel appears as a coherent, expanding cylindrical structure of length >120 pc and and width 35-50 pc and the walls maintain an approximately constant (but subsonic) expansion velocity of ~60 km/s. We use the channel to examine further the relationship between the narrow and broad component emitting gas within the inner wind. Within the starburst energy injection zone, we find that turbulent motions (as traced by the broad component) appear to play an increasing role with height.