Low density ionized gas associated with M17, G333.3-0.4 and RCW74

We made observations in the H166 ∞ line and in the 1.4 GHz continuum at several positions in extended ionized gas associated with the H II regions M17, G333.3 - 0.4, and RCW 74, respectively. We derived electron temperatures and other physical parameters of these regions and compared them with those obtained by other authors from higher frequency radio recombination line observations. The results are compatible with a description of relatively extended H II regions, each consisting of a small compact region (Nsub(e)=100 to 1000cm-3) embedded in an extended ionizing gas envelope of lower density (Nsub(e)=1-10cm-3). Electron temperatures appear to vary with density, at least in M17. (authors)