3.1. The Milky Way

By far the closest case of a large-scale outflow is the wind in our own Galaxy. Evidence for a dusty bipolar wind extending ~ 350 pc (~ 1°) above and below the disk of our Galaxy has recently been reported by Bland-Hawthorn & Cohen (2003) based on data from the Midcourse Space Experiment (MSX). The position of the warm dust structure coincides closely with the well-known Galactic Center Lobe detected at radio wavelengths (e.g., Sofue 2000 and references therein). Simple arguments suggest that the energy requirement for this structure is of order ~ 10⁵⁵ ergs with a dynamical time scale of ~ 1 Myr.

Bland-Hawthorn & Cohen (2003) also argue that the North Polar Spur, a thermal X-ray/radio loop that extends from the Galactic plane to b = + 80° (e.g., Sofue 2000), can naturally be explained as an open-ended bipolar wind, when viewed in projection in the near field. This structure extends on a scale of 10 - 20 kpc and implies an energy requirement of ~ (1 - 30) × 10⁵⁵ ergs and a dynamical timescale of ~ 15 Myr, i.e. considerably longer than that of the smaller structure seen in the MSX maps. If confirmed, this may indicate that the Milky Way Galaxy has gone through multiple galactic wind episodes. Bland-Hawthorn & Cohen (2003) point out that the North Polar Spur would escape detection in external galaxies; it is therefore possible that the number of galaxies with large-scale winds has been (severely?) underestimated.