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, H168α, H167α, and H166α 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.