Diffuse radio recombination line emission on the Galactic plane between l=36° and 44°

Radio recombination lines (RRLs) can be used to determine the emission measure unambiguously along the Galactic plane. We use the deep (2100 s per beam) H I Parkes Zone of Avoidance (ZOA) survey which includes three RRLs (H166 alpha, H167 alpha and H168 alpha) within its bandwidth. The region l = 36 degrees to 44 degrees, b = - 4 degrees to +4 degrees is chosen to include emission from the Local, Sagittarius and Scutum arms. An 8 degrees x 8 degrees data cube centred at (l, b) = (40 degrees, 0 degrees) is constructed of RRL spectra with velocity and spatial resolution of 27 km s(-1) and 15.5 arcmin, respectively. Well-known H II regions are identified as well as the diffuse RRL emission on the Galactic plane. A Galactic latitude section of the integrated RRL emission across the Galactic plane delineates the brightness temperature (T(b)) distribution which has a half-power width in latitude of similar or equal to 1 degrees.5. A value of the electron temperature T(e) similar or equal to 8000K is derived from a comparison with the Wilkinson Microwave Anisotropy Probe (WMAP) free-free maximum entropy method. The Tb distribution from the present RRL data is combined with the WMAP 5-yr data to derive the latitude distribution of the residual 'anomalous' emission on the Galactic ridge. In this paper, we demonstrate that diffuse ionized emission on the Galactic ridge can be recovered using RRLs from the ZOA survey. This method is therefore able to complement the H alpha data at low Galactic latitudes, to enable an all-sky free-free template to be derived.