NH₃ and H₂O Emission in our Galaxy

Extract

In 1968 NH₃ molecules were discovered in gas clouds near the center of our Galaxy. The NH₃ molecules showed emission lines in transitions arising from metastable J = K rotational states. NH₃ has many transitions at about 1.25 cm; hence, a great deal can be learned about the gas clouds. In particular, 1-1, 2-2, 3-3, 4-4 have excitation temperatures of 24K, 65K, 125K, 203K respectively. Thus, one can determine temperatures very accurately by comparing line strengths to statistical theory. NH₃ is particularly useful as an interstellar thermometer since all of these lines can be detected by a single radio telescope with essentially the same beam size. Generally, a telescope measures the antenna temperature T_A of an object which is related to the brightness temperature T_B of an object by T_A=Ω_s/Ω_A T_Bj; Ω_s/Ω_A≤1. T_A = T_B when Ω_s≥Ω_A where Ω_A is the solid angle of the antenna pattern and Ω_s, is the solid angle of the source as seen from the antenna. If measurements can be made with the same telescope, T_B for different spectral lines will be simply related to the T_A for the lines and one does not have the complication of source size and geometry.