Assume there are only two phases in the interstellar medium, both atomic. The first is the cold neutral medium (CNM), characterized by a kinetic temperature of 80K and the 21cm hydrogen line width (FWHM) of 2 km/s. The other phase is the warm neutral medium (WNM) and assume that it has a kinetic temperature of 8000 K and is characterized by a line width (FWHM) of 20 km/s. Assuming no turbulence, answer the following questions: (a) What column density of HI produces a line center optical depth in the 21-cm line of unity for both the CNM and the WNM phases? (b) Measurements of HI towards distance radio sources have shown a typical optical depth due to the CNM phase is 0.5. If the density of the CNM phase is 100 , estimate the typical size and mass of these CGM clouds.

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