Abstract

Microwave inversion radiation from one or more of the metastable rotational levels of ammonia (NH2) has been observed in several galactic objects including W3 (OH), Orion A, W43, WS1, DR-21 (OH), and five sources in the galactic center region (including two previously known). The NH2 emission is extended in most of these sources. NH3 has also been tentatively detected in cloud 4, a dark dust cloud, implying a lower limit of S x 10 cm-3 to the particle density in this source. The relative antenna temperatures of lines from the (J, K) = (1, 1), (2, 2), and (3, 3) rotational levels in four sources indicate that the excitation is not adequately explained by thermalization at a single temperature. Several possible explanations are discussed, including the effects of a core-halo structure for the sources, and an anomalous ortho/para NH2 abundance ratio. We have also investigated radiation from the nonmetastable (2, 1) level in Sgr B2. An angular area ofS.2 + 1.5 square arc minutes was found for this source. Ifthe average H2 density is 106 cm-3 in a spherical source of this projected size at the distance of the galactic center, the total mass would be 10 M0. The density of particles required to collisionally populate the upper levels of the metastable inversion doublets so that emission lines can be seen above the continuum background is estimated to be 10 . Ammonia was detected in 11 out of 20 sources searched to a limit of 0.25 K, indicating that it is a fairly widespread molecule in the interstellar medium and that these relatively high density conditions are quite common in the Galaxy. A search for the molecule PN was unsuccessful. This may have significant implications about the dominant form assumed by nitrogen in dark interstellar clouds. Subject headings: galactic nuclei - molecules, interstellar nebulae - radio lines