THE [N-II] 205-MICRON LINE IN M82 - THE WARM IONIZED MEDIUM

Detection of the 205 mum fine structure line of N II in the nearby sta rburst galaxy M82 is reported. The intensity within a 54'' (FWHM) beam is (7.1 +/- 1.2) X 10(-19) W cm-2. The ratio of the intensity of the recently detected 122 mum line to that of the 205 mum line is =4.2(-1. 2)+1.6, significantly larger than the corresponding Galactic value of 1.6 +/- 0.3, reflecting higher electron densities within the central 8 50 pc of M82 in comparison to the COBE Galactic average. The 205 mum l ine profile is consistent with other far-infrared fine-structure line profiles observed in M82. The observations are interpreted in the cont ext of a two-component model of the ionized medium in M82. We find tha t a component of density as low as approximately 50 cm(-3) can compris e up to 70% of the total mass of warm ionized gas within the beam. The balance of the ionized mass is comprised of a component of density gr eater than or similar to 100 cm-3. A model is explored in which the de nser ionized medium constitutes the boundaries of neutral surfaces whi ch border the expanding hot plasma from the nuclear region.