The HIPASS survey of the Galactic plane in Radio Recombination Lines
  We present a Radio Recombination Line (RRL) survey of the Galactic Plane from
the HI Parkes All-sky Survey and associated Zone of Avoidance survey, which
mapped the region l=196degr -- 0degr --52degr and |b| < 5degr at 1.4 GHz and
14.4 arcmin resolution. We combine three RRLs, H168$\alpha$, H167$\alpha$, and
H166$\alpha$ to derive fully sampled maps of the diffuse ionized emission along
the inner Galactic plane. The velocity information, at a resolution of 20 km/s,
allows us to study the spatial distribution of the ionized gas and compare it
with that of the molecular gas, as traced by CO. The longitude-velocity diagram
shows that the RRL emission is mostly associated with CO gas from the molecular
ring and is concentrated within the inner 30degr of longitude. A map of the
free-free emission in this region of the Galaxy is derived from the
line-integrated RRL emission, assuming an electron temperature gradient with
Galactocentric radius of $496\pm100$ K/kpc. Based on the thermal continuum map
we extracted a catalogue of 317 compact (<15 arcmin) sources, with flux
densities, sizes and velocities. We report the first RRL observations of the
southern ionized lobe in the Galactic centre. The line profiles and velocities
suggest that this degree-scale structure is in rotation. We also present new
evidence of diffuse ionized gas in the 3-kpc arm. Helium and carbon RRLs are
detected in this survey. The He line is mostly observed towards HII regions,
whereas the C line is also detected further away from the source of ionization.
These data represent the first observations of diffuse C RRLs in the Galactic
plane at a frequency of 1.4 GHz.
