Anomalous ammonia absorption in DR 21

Ammonia has been detected for the first time in absorption, against the strong radio continuum source DR 21, and the inferred optical depth is further evidence for clumping. Some of the hyperfine components are observed in emission and some in absorption, indicating deviations of the level populations on the order of a few percent from LTE. An infrared trapping model requires that the cloud be fragmented into clumps of size less than 0.2 pc with velocity widths of about 0.3 km s-1