(a) Suppose a uniform, extended HI cloud has a physical temperature of TK D
2:73 K. If the only background source is the 2.73 K microwave background,
would you expect to observe the HI line in emission or absorption or no line
radiation at all?
(b) Repeat if there is a background source with main beam brightness temperature,
TMB D 3 K, that is, TMB > TK. What would be the temperature of the
absorption, TL, in K if
D 1?
(c) Repeat for TK D 3:5 K.