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 H I Parkes All-sky Survey and associated Zone of Avoidance survey, which mapped the region l = 196�?-0�?-52�? and |b| ?� 5�? at 1.4 GHz and 14.4 arcmin resolution. We combine three RRLs, H168?�, H167?�, and H166?� 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 -1 , 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 30�? 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 �� 100 K kpc -1 . 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 H II 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.