1) 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 80 K and the 21 cm hydrogen
line width (FWHM) of 2 km/s. The other phase is the warm neutral medium (WNM) and assume
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?
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 cm-3, estimate the typical size and
mass of these CGM clouds.
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