Detection of highly-ionized diffuse gas in the Galactic plane^*

¹ Department of Astronomy, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan
² Department of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan

Corresponding author: T. Onaka, onaka@astron.s.u-tokyo.ac.jp

Received: 1 September 2000
Accepted: 5 November 2001

Abstract

We obtained maps of the central 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 [] 52, 88 μm, [] 57 μm, and [] 122 μm lines. These ionized lines have been detected in the entire observed area, not only in the optically bright region, but also in the molecular cloud direction and the region surrounding the Carina nebula. The electron density was derived from the [] 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 regions, Car I and Car II. The other is an extended diffuse component, in which the ratio of the [] lines is nearly at the low electron density limit ( cm^-3). The observed []/[] 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.