Observations of low frequency recombination lines

Abstract

Recombination lines have been detected in the radio part of the electromagnetic spectrum over a frequency range from about 200 MHz to 30 GHz. This corresponds to a range in principal quantum number, n, of n ∼ 300 to n ∼ 60. As well as emission from ionized hydrogen recombination lines have also been detected from heavier elements such as helium, carbon and Silicon. Most of the recombination lines detected originate in hot gas, with an electron temperature Te ∼ 10³ -10⁴ K, which is associated with HII regions. At high frequencies the combination of the physics of recombination line emission and the fact that radio telescopes have smaller half-power beam-widths result in the observations being more sensitive to high density, high emission measure, small diameter sources such as compact HII regions. Conversely, at low density, low emission measure, large diameter sources such as extended HII regions. For the purpose of this article I shall consider only a transitions (Δn=1) from hydrogen and further that low frequency recombination lines are those which is close in frequency to the 21 cm line of neutral hydrogen. For reasons that will be given below this frequency at which the division is made is not as arbitrary as it may first seem.