NH 3 ( 1 , 1 ) survey toward southern hemisphere HII regions . I

The (J,K) = (1,1) metastable ammonia line was searched for in the direction of more than one hundred southern HII regions located between 270◦and 15◦ in galactic longitude, resulting in new detections toward 21 of them. Ammonia detection rate was about 30 % towards all directions except in the longitude interval 270◦–290◦ where no ammonia lines were found in the 15 selected HII regions. A high detection rate (≥ 0.6) was observed toward positions where IRAS point sources with color indices of ultra–compact HII regions were in the beam of the telescope. An anticorrelation between the number of detected NH3 sources and the distance to the Galactic Center was found, compatible with the observed gradient in N abundance. From the value of this gradient and the detection rate of ammonia lines toward HII regions, it is inferred that the number of ammonia sources must be proportional to the inverse of the column density of the molecular cloud.