The N (II) 205 micron line in M82: The warm ionized medium

Abstract

Detection of the 205 micron fine structure line of N II in the nearby starburst 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 micron line to that of the 205 micron 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 850 Pc of M82 in comparison to the COBE Galactic average. The 205 micron line profile is consistent with other far-infrared fine-structure line profiles observed in M82. The observations are interpreted in the context of a two-component model of the ionized medium in M82. We find that a component of density as low as ~50 cm^-3^ can comprise 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 >~ 100 cm^-3^. A model is explored in which the denser ionized medium constitutes the boundaries of neutral surfaces which border the expanding hot plasma from the nuclear region.