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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