Detection of highly-ionized diffuse gas in the Galactic plane

Abstract

We obtained maps of the central 40′ × 20′ region of the Carina nebula, an active star forming region in the southern Milky Way, in various emission lines in the spectral range 43-197 μm with the LWS on board the ISO. This paper reports on the results of [O III] 52, 88 μm, [N III] 57 μm, and [N II] 122 μm lines. These ionized lines have been detected in the entire observed area, not only in the optically bright H II region, but also in the molecular cloud direction and the region surrounding the Carina nebula. The electron density was derived from the [O III] lines and two distinct components were seen in the electron density map. One is a component with a density of 100-350 cm-3, which encloses two H II regions, Car I and Car II. the other is an extended diffuse component, in which the ratio of the [OIII] lines is nearly at the low electron density limit (ne < 100 cm-3). The observed [N III]/[O III] line intensity ratio also supports low electron density of the extended gas component. The diffuse component was detected in the entire observed region, which corresponds to ∼30 pc at the distance of the Carina nebula. The present LWS observations of the ionic lines indicate the existence of highly-ionized diffuse gas of low electron density extending around the Carina nebula.