Rotational diagram of ammonia in M 51. The horizontal axis is the upper state energy, and the vertical axis corresponds to the abundances (g_Ju = 2J_upper + 1, g_I is the spin statistical weight). The (4, 4) line is regarded as an upper limit, because it is barely detected. The error of each data point was estimated based on the noise level of each spectrum. About the rotational diagram analysis, see the section of results and references there.