Radio recombination line observations at 1.0-1.5GHz with FAST

HII regions made of gas ionized by radiations from young massive stars, are widely distributed in the Milky Way. They are tracers for star formation, and their distributions are correlated with the Galactic spiral structure. Radio recombination lines (RRLs) of hydrogen and other atoms allow for the most precise determination of physical parameters such as temperature and density. However, RRLs at around 1.4 GHz from H II regions are weak and their detections are difficult. As a result, only a limited number of detections have been obtained yet. The 19-beam receiver on board of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) can simultaneously cover 23 RRLs for Hn alpha, Hen alpha, and Cn alpha (n = 164 - 186), respectively. This, combined with its unparalleled collecting area, makes FAST the most powerful telescope to detect weak RRLs. In this pilot survey, we use FAST to observe nine H II regions at L band. We allocate 20 minutes pointing time for each source to achieve a sensitivity of around 9 mK in a velocity resolution of 2.0 kms. In total, 21 RRLs for Hn alpha and Cn alpha at 1.0 - 1.5 GHz have been simultaneously detected with strong emission signals. Overall, the detection rates for the H167 alpha and C167 alpha RRLs are 100%, while that for the He167 alpha RRL is 33.3%. Using hydrogen and helium RRLs, we measure the electron density, electron temperature, and pressure for three H II regions. This pilot survey demonstrates the capability of FAST in RRL measurements, and a statistically meaningful sample with RRL detection, through which knowledge about Galactic spiral structure and evolution can be obtained, is expected in the future.