The observable properties of cool winds from galaxies, AGN, and star clusters -- II. 3D models for the multiphase wind of M82

Abstract

Galactic winds are a crucial player in galaxy formation and evolution, but observations of them have proven extraordinarily difficult to interpret, leaving large uncertainties even in basic quantities such as mass outflow rates. Part of this uncertainty arises from the relatively simplistic models to which complex wind observations are often fit, which inevitably discard much of the available information. Here we present an analysis of the wind of the nearby dwarf starburst galaxy M82 using a semi-analytic model that is able to take advantage of the full three-dimensional information present in position-position-velocity data cubes measured in the Hi 21 cm line, the CO 2-1 line, and the Ha line. Our best-fitting model produces position-dependent spectra in good agreement with the observations, and shows that the total wind mass flux in the atomic and molecular phases is approx 10 M_sun yr-1 (corresponding to a mass loading factor of about 2 - 3), with less than a factor of two uncertainty; the mass flux in the warm ionised phase is more poorly constrained, and may be comparable to or smaller than this. At least over the few kpc off the plane for which we trace the outflow, it appears to be a wind escaping the galaxy, rather than a fountain that falls back. Our fits require that clouds of cool gas entrained into the wind expand only modestly, suggesting they are magnetically confined. Finally, we demonstrate that attempts to model the wind using simplifying assumptions such as instantaneous acceleration and a constant terminal wind speed can yield significantly erroneous results.