M. Mizutani 1, T. Onaka 1, & H. Shibai 2
1 Department of Astronomy, School of Science, University of Tokyo, Japan
2 Department of Physics, School of Science, Nagoya University, Japan
We have made mapping observations of the Carina Nebula, a very active
star-forming region on the South Galactic plane. The observations were
made with LWS01 (full grating scan mode). The observations have been
made for about 40'x20' of the central region of the nebula;
.
The spatial separation between the observed
points was 3'. We have obtained the spectrum from 45 to 170
m at
each point. In the spectra, several fine structure lines such as [OI]
63, 145m, [OIII] 52, 88m, [NIII] 57m, [NII] 121m,
and [CII] 158m have clearly been detected.
The [OIII] lines, emitted from the ionized gas, have been detected at all of the observed positions. The intensity distributions of the [OIII] lines show the presence of highly excited ionized gas around Car I and Car II. However, these lines have also been detected even at the positions where molecular clouds exist, indicating the presence of diffuse ionized gas around the Carina Nebula.
The electron density has been derived from the two [OIII] lines. For the central bright ionized region, the derived electron density is in the range 100 - 400 cm-3. It is lower than those of previously observed HII regions, 102 - 104 cm-3. This may indicate that the Carina Nebula is an evolved HII region. For the extended diffuse ionized gas, the electron density is esimated to be about 30 cm-3. Even taking account of the calibration errors, an upper limit of the density is 60 cm-3.
The size in the line of sight direction can be estimated from the line intensity and it is about 10pc for the central bright region, which is simlar to the extension of this nebula on the sky. The size for the diffuse ionized gas is estimated to be about 1-2 kpc. This is compatible with the geometrical distance of the observed region on the Carina arm. Thus the present observations indicate the presence of diffuse ionized gas of low electron density over the Galactic plane. This ionized gas is considered to correspond to the extremely low density (ELD) ionized gas, which had been discussed previously based on radio observations.
We have detected the ELD ionized gas around the Carina Nebula by direct observations and derived the electron density unambiguously for the first time. Using the electron density, we can also estimate the fraction of the energy emitted by the extended ionized gas in the form of [OI] and [CII] fine structure lines.