• radiation mechanisms: general;
  • methods: data analysis;
  • ii regions;
  • ISM: lines and bands;
  • Galaxy: structure;
  • radio lines: ISM


We present the derivation of the free–free emission on the Galactic plane between ℓ= 20° and 44° and |b|≤ 4°, using radio recombination line (RRL) data from the H i Parkes All Sky Survey (HIPASS). Following an upgrade of the RRL data reduction technique, which improves significantly the quality of the final RRL spectra, we have extended the analysis to three times the area covered in Alves et al. The final RRL map has an angular resolution of 14.8 arcmin and a velocity resolution of 20 km s−1.

The electron temperature (Te) distribution of the ionized gas in the area under study at 1.4 GHz is derived using the line and continuum data from the present survey. The mean Te on the Galactic plane is 6000 K. The first direct measure of the free–free emission is obtained based on the derived Te distribution. Subtraction of this thermal component from the total continuum leads to the first direct measurement of the synchrotron emission at 1.4 GHz. A narrow component of width 2° is identified in the latitude distribution of the synchrotron emission. We present a list of H ii regions and supernova remnants (SNRs) extracted from the present free–free and synchrotron maps, where we confirm the synchrotron nature of the SNRs G42.0−0.1 and G41.5+0.4 proposed by Kaplan et al. and the SNR G35.6−0.4 recently re-identified by Green.

The latitude distribution for the RRL-derived free–free emission shows that the Wilkinson Microwave Anisotropy Probe (WMAP) maximum entropy method is too high by ∼50 per cent, in agreement with other recent results. The extension of this study to the inner Galaxy region ℓ=−50° to 50° will allow a better overall comparison of the RRL result with WMAP.