Analysis of the thin layer of Galactic warm ionized gas in the range 20° <l < 30° -1.5° < b < 1.5°

We present an analysis of the thin layer of Galactic warm ionized gas at an angular resolution ~10 arcmin. This is carried out using radio continuum data at 1.4, 2.7 and 5 GHz in the coordinate region . For this purpose, we evaluate the zero level of the 2.7- and 5-GHz surveys using auxiliary data at 2.3 GHz and 408MHz. The derived zero-level corrections are T_zero(2.7GHz) = 0.15 +/- 0.06K and T_zero(5GHz) = 0.1 +/- 0.05K. We separate the thermal (free-free) and non-thermal (synchrotron) component by means of a spectral analysis performed adopting an antenna temperature spectral index -2.1 for the free-free emission, a realistic spatial distribution of indices for the synchrotron radiation and by fitting, pixel-by-pixel, the Galactic spectral index. We find that at 5 GHz, for |b| = 0°, the fraction of thermal emission reaches a maximum value of 82 per cent, while at 1.4 GHz, the corresponding value is 68 per cent. In addition, for the thermal emission, the analysis indicates a dominant contribution of the diffuse component relative to the source component associated with discrete HII regions.