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The first astronomical detection of the metastable ($J,K$) = (18,18) line ofNH3 is reported. With 3130 K above the ground state, this is the NH3 line withby far the highest energy detected in interstellar space. It is observed inabsorption toward the galactic center star forming region Sgr B2. There is aclear detection toward Sgr B2(M) and a likely one toward SgrB2(N). An upperlimit for emission is determined for Orion-KL. If we combine the (18,18) lineresults from Sgr B2(M) with the previously measured (12,12) absorption line, wefind a rotation temperature of >1300 K for the absorbing cloud. This is atleast a factor of two higher than previously derived values from less highlyexcited ammonia lines, giving a lower limit to the kinetic temperature. Thereis a hot low density gas component in the envelope of SgrB2. It is possiblethat the (18,18) line arises in this region. The radial velocity of the lowdensity, hot envelope is the same as that of the dense hot cores, so the(18,18) line could also arise in the dense hot cores where non-metastable (J>K)absorption lines from energy levels of up to 1350 K above the ground state havebeen observed. A discussion of scenarios is presented.Comment: 6 pages, 2 figures, 2 Tables, accepted for publication in A&A Main  Journa

The detection of the (J,K) = (18, 18) line of NH3

The detection of the (J,K) = (18, 18) line of NH₃