The inner parts of our galaxy may be a dramatic example showing how small scale fluctuations can act in concert to drive large scale flows. The figure shows x-ray emission from the inner galaxy, emitted by gas at several million degrees. The gas was probably heated over millions of years by multiple supernova explosions, and in previous models it has been assumed to be at rest, supported against the galactic gravitational field by its own pressure. However, supernovae also create the relativistic nuclei known as cosmic rays. As the cosmic rays stream away from the supernovae, they excite short wavelength magnetic fluctuations, which transfer momentum to the gas. We showed that this process can impart enough momentum to actually drive the gas out of the galaxy. This galactic wind carries away about 2 solar masses of gas each year, making it important for galactic evolution. The wind model is a better fit to current observations than the static model, and we are in the process of developing additional observational tests based on the synchrotron and gamma radiation expected from cosmic rays in the wind. Cosmic ray driven winds of this type may also have been important in young galaxies, and for heating the intergalactic medium.

Soft x-ray emission from the inner Milky Way galaxy, showing the presence of hot (several million degree) gas. Figure from J. Everett, E. Zweibel, R. Benjamin, D. McCammon, L. Rocks, J. Gallagher, “The Milky Way’s Kiloparsec-Scale Wind,” Astrophysical Journal, V. 674, 258 (2008).

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