Zeeman effect in atomic hydrogen. (a) Idealized situation.The line shift is a very small fraction of the linewidth. However, opposite polarizations are shifted in opposite direc-tions.When we subtract one polarization from the other, we are left with a very distinctive pattern. (Note that the difference curve is approximately the derivative of the original curve.This follows directly from the definition of a derivative.) (b) Spectra from a real source. In this figure, the two smooth curves that have very little noise are the spectra for opposite polarizations.The difference is shown on an expanded scale, so it looks noisy.The dashed line is the best fit to that difference spectrum. [(b)Carl Heiles (University of California at Berkeley)/Heiles, C., Astrophys.J., 336, 808, 1989, Fig. 1a]
We have already seen (in Chapter 6) that some energy levels shift in the presence of a magnetic field, the Zeeman effect. The levels involved in the 21 cm line fall into this category. The stronger the magnetic field, the greater the shift. This means that we can use the Zeeman shift in the 21 cm line to measure the strength of interstellar magnetic fields. The experiment is difficult because the Zeeman shift is much less than the width of the normal 21 cm line. However, opposite polarizations are shifted in opposite directions. Since we can detect different polarizations separately, we can subtract one polarization's spectrum from the other, leaving a very small signal, as shown in Fig. 14.13. The experiment is also difficult because a small difference in the response of the telescope to the two polarizations can mimic the effects of a Zeeman shift. Despite these difficulties, recent experiments have succeeded in measuring fields of the order of tens of microgauss in a growing number of interstellar clouds. Fields of this strength may sound very weak, but they are strong enough to influence the evolution of these clouds, as we will discuss in the next chapter.
By making maps of the 21 cm emission astronomers have been able to form a good picture of the cloud structure in the interstellar gas. These maps show an irregular cloud structure, similar to that shown in the dust clouds. Typical clouds have the following physical parameters: temperature, 100 K; hydrogen density nH ~ 1-10 cm"3, lengths of tens of parsecs; hydrogen column densities up to ~ 1021 cm"2. The clouds fill about 5% of the volume of interstellar space, meaning that the average density of atomic hydrogen in interstellar space is of the order of 0.1 cm"3; One interesting recent finding is the presence of large HI shells, which stir up the interstellar medium throughout the galaxy (Fig. 14.14).
The regions between the clouds are not empty. Studies of the line profiles of the 21 cm line show very broad, faint wings. This is interpreted as coming from a small amount of very hot gas. Temperatures of about 104 K have been estimated for this low density gas between the clouds. We will see later in this chapter that the low density means it is very hard for the gas to lose energy
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