| Everyone who has been at ASTRON or JIVE for long enough, has probably heard one astronomer or another exclaim at some time that magnetic fields are an important part of the Universe and should not be ignored.
Here I show one of the reasons why we tend to assert this. This graph describes the energy density of various components of the Milky Way, as a function of the distance from the Galactic center. The energy enclosed in the magnetic field component (represented by large and small asterisks for different ways of calculating) is seen to be a dominant factor: the magnetic field and the turbulent gas in the Milky Way contain similar amounts of energy (various literature estimates of the turbulent gas energy density are shown). At the solar radius of about 8.5 kpc, the one measurement of the thermal energy content of the gas appears to be rather insignificant.
The fact that the energy contained in the turbulent gas and the Galactic magnetic field are comparable means that the two components influence each other significantly. E.g. if shock waves compress the gas, the magnetic field will not be just a passive tracer following the gas motions, but instead will provide significant feedback which in turn changes the gas dynamics. This rough equipartition holds only when considering the Milky Way as a whole, or at least a large chunk of it. On small scales, e.g. where stars form, the ratio of energy densities may be very different.
The beautiful background picture is a composite of gas and dust in the Galactic plane observed as part of the Canadian Galactic Plane Survey. Synchrotron radiation shows up as purplish-red and yellow, e.g. the circular supernova remnant in the right upper corner. Reddish point-like sources scattered throughout the image are background radio galaxies and quasars. Bright compact, blue-white nebulae reveal the presence of a newly-formed, hot star embedded in a dense cocoon of heated gas. Greenish emission shows the presence of diffuse clouds of ionized gas, while the dark blue emission seen throughout the entire region is radiation from heated dust particles. Magnetic fields have helped shape this medium. Details can be found in Haverkorn (2010), arXiv:1012.3755.
|Haverkorn (2010). Background picture composed for the CGPS by Jayanne English (U. Manitoba) with the support of Russ Taylor (U. Calgary)