What are the Radial Distributions of Density, Outflow Rates, and Cloud Structures in the M 82 Wind?

TL;DR

This study measures the radial distribution of density, outflow rates, and cloud properties in M 82's galactic wind, revealing minimal loss of mass and energy over several kiloparsecs and challenging existing theoretical models.

Contribution

First spatially-resolved measurements of outflow properties in M 82, linking emission and absorption features, and providing new insights into wind feedback mechanisms.

Findings

Electron density drops from 200 to 40 cm$^{-3}$ with radius.

Mass, momentum, and energy outflow rates remain nearly constant over 2.2 kpc.

Cloud pressures and densities exceed recent theoretical model predictions.

Abstract

Galactic winds play essential roles in the evolution of galaxies through the feedback they provide. Despite intensive studies of winds, the radial distributions of their properties and feedback are rarely observable. Here we present such measurements for the prototypical starburst galaxy, M 82, based on observations by Subaru telescope. We determine the radial distribution of outflow densities ($n_e$) from the spatially-resolved [S II] $\lambda\lambda$ 6717, 6731 emission-lines. We find $n_e$ drops from 200 to 40 cm$^{-3}$ with radius ($r$) between 0.5 and 2.2 kpc with a best-fit power-law index of $r^{-1.2}$. Combined with resolved H$\alpha$ lines, we derive mass, momentum, and energy outflow rates, which drop quite slowly (almost unchanged within error bars) over this range of $r$. This suggests that the galactic wind in M 82 can carry mass, momentum, and energy from the central regions to a few kpc with minimal losses. We further derive outflow cloud properties, including size and column densities. The clouds we measure have pressures and densities that are too high to match those from recent theoretical models and numerical simulations of winds. By comparing with a sample of outflows in local star-forming galaxies studied with UV absorption-lines, the above-derived properties for M 82 outflows match well with the published scaling relationships. These matches suggest that the ionized gas clouds traced in emission and absorption are strongly related. Our measurements motivate future spatially resolved studies of galactic winds, which is the only way to map the structure of their feedback effects.