Cosmic Rays and Galactic Winds

The accompanying figure shows the inner Milky Way in soft x-rays. It is indicative of  hot gas, probably heated by supernovae - the star formation rate, and hence the supernova rate, are very high in the inner Galaxy. Supernovae are known to  accelerate cosmic rays, so cosmic rays also must be present. We made a model of a wind from the inner galaxy which is driven by gas pressure and cosmic ray pressure, in roughly equal amounts (Everett et al. 2008, 2010) which is able to fit the observations.

How does cosmic ray pressure actually drive thermal gas? This is a bit subtle. Cosmic rays, after all, are relativistic particles. They should just be able to stream out of the galaxy at the speed of light. However, it turns out that  a large flux of cosmic rays streaming through a magnetized plasma leads to the growth of magnetic fluctuations. These fluctuations extract momentum and energy from the cosmic rays, and transfer it to the gas. The end result is that the cosmic rays and thermal gas are strongly coupled together. This works very nicely in hot, fully ionized gas. It doesn't work well at all, however, in cool interstellar clouds (Everett & Zweibel 2011), because the fluctuations are rapidly dissipated in cold gas. This means that the cooler gas seen in outflows from other galaxies must be driven by another mechanism, or must have formed once the wind had already been accelerated.

We are applying similar ideas to other galaxies, where conditions can be quite different. The starburst region of M82, where the cosmic ray density, gas density, and magnetic fieldstrength are all very high, is a good example. Here, the wind may be an evaporative flow.