Velocity Vr (km s_1)

Figure 2.2 Radio spectrum of a quasar partially obscured by HI gas. The upper solid curve shows, as a function of velocity (or, equivalently, wavelength), the intensity of emission in the 21 cm line. The lower solid curve shows, with a different amplitude scale, the absorption. This absorption is mostly confined in velocity between the two vertical bars. Finally, the upper dashed curve indicates the contribution to the total emission from the warm neutral medium.

evidence for extended, dark halos. The distribution of HI surface density in our own Galaxy is shown in Figure 2.3. Within the central bulge, the density is very low, but rises to a fairly uniform level between radii of 4 and 14 kpc. At larger radii the surface density declines, but in a manner that is rather uncertain because of the poorly known Galactic rotation at these distances.

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