The first VLBI measurement of interstellar ammonia (NH3) masers is reported. The positions, as well as size and brightness temperature limits, for the masers in the NH3 (9, 6) inversion transition that are associated with the ultracompact H II regions W51 e1 and e2 were obtained. The masers arise from two positions, each of which is located close to but not coincident with a radio continuum peak. The NH3 masers are closer to the H2O masers than to the OH masers or the ultracompact H II regions. However, at least one of the two NH3 maser spots mapped seems offset from the region of H2O maser activity. The NH3 maser spots are unresolved in angle and smaller than 0.1 milli-arcsec. Unless these masers are highly beamed, they appear to be saturated.
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Pratap et al. (1991) studied this question.