ANOMALOUSLY EXCITED OH AND COMPETITION BETWEEN MASER TRANSITIONS TOWARD CENTAURUS-A

The ground-state radio transitions of OH have been observed toward the nucleus of the nearby active galaxy Centaurus A, using the Australia Telescope Compact Array. The main lines at rest frequencies 1667 and 1 665 MHz show molecular absorption at velocities similar to those obser ved for other molecular species at millimeter wavelengths, which indic ates that the lines of sight at radio and millimeter wavelengths sampl e the same material. The satellite transitions at 1612 and 1720 MHz ex hibit stimulated emission and absorption, including the first extragal actic detection of maser emission in the 1720 MHz line. A remarkable f eature of the two satellite transitions is that they are conjugate for all velocity components, with one in emission and the other equally s trong in absorption. This is explained as the competition of the two l ines for the same inverting infrared radiation in the regime in which the infrared lines are just becoming optically thick. The transition f rom 1720 MHz to 1612 MHz maser emission depends critically on the OH c olumn density and therefore yields direct information on the distribut ion of molecular gas.