Diffuse radio recombination line emission on the Galactic plane between ℓ=36° and44°

Abstract

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 (H166α, H167α and H168α) within its bandwidth. The region � = 36 ◦ to 44 ◦ , b =− 4 ◦ to +4 ◦ is chosen to include emission from the Local, Sagittarius and Scutum arms. An 8 ◦ × 8 ◦ data cube centred at (� , b) = (40 ◦ ,0 ◦ )i s 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 � 1. ◦ 5. A value of the electron temperature T e � 8000 K is derived from a comparison with the Wilkinson Microwave Anisotropy Probe (WMAP) free–free maximum entropy method. The T b 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 Hα data at low Galactic latitudes, to enable an all-sky free–free template to be derived.