Diffuse RRL emission on the Galactic plane between l=36 to 44 degrees
  Radio recombination lines (RRLs) can be used to determine the emission
measure unambiguously along the Galactic plane. We use the deep (2100s per
beam) HI Parkes Zone of Avoidance survey which includes 3 RRLs (H$166\alpha$,
H$167\alpha$ and H$168\alpha$) within its bandwidth. The region $\ell =
36\degr$ to $44\degr$, $b = -4\degr$ to $+4\degr$ is chosen to include emission
from the Local, Sagittarius and Scutum arms. An $8\degr \times 8\degr$ data
cube centred at $(\ell, b) = (40\degr, 0\degr)$ is constructed of RRL spectra
with velocity and spatial resolution of 27$\kms$ and 15.5 arcmin, respectively.
Well-known \hii 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 $\simeq 1\fdg5$.
  A value of the electron temperature $T_{e} \simeq 8000$ K is derived from a
comparison with the WMAP free-free MEM model. The $T_{b}$ distribution from the
present RRL data is combined with the WMAP 5-yr data to derive the anomalous
dust 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 \ha data at low Galactic latitudes, to enable an all-sky
free-free template to be derived.
