equation (2.11) that hHI is 10 pc, only 10 percent of the observed value! In hindsight, this failure is not too surprising. After all, 21 cm observations show that the cold gas actually consists of discrete clouds in seemingly random motion. A typical cloud velocity in the z-direction is 6 km s-1. Substitution of this value for cHI in (2.11) yields a scale height much closer to the observed one. The lesson here, as illustrated in Figure 2.6, is that the vertical distribution of cold HI gas stems from the bulk motion of individual clouds rather than internal pressure support. The salient question is now the energizing agent for this motion.

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