Spectroscopy with the Engineering Development Array: cold H+ at 63 MHz towards the Galactic Centre

 The Engineering Development Array (EDA) is a single test station for Square Kilometre Array (SKA) precursor technology. We have used the EDA to detect low-frequency radio recombination lines (RRLs) from the Galactic Centre region. Low-frequency RRLs are an area of interest for future low-frequency SKA work as these lines provide important information on the physical properties of the cold neutral medium. In this project, we investigate the EDA, its bandpass, and the radio frequency interference environment for low-frequency spectroscopy. We present line spectra from 30 to 325 MHz for the Galactic Centre region. The decrease in sensitivity for the EDA at the low end of the receiver prevents carbon and hydrogen RRLs to be detected below 40 and 60 MHz, respectively. RFI strongly affects frequencies in the range 276–292, 234–270, 131–138, 95–102, and below 33 MHz. Cnα RRLs were detected in absorption for quantum levels n = 378–550 (39–121 MHz) and in emission for n = 272–306 (228–325 MHz). Cnβ lines were detected in absorption for n = 387–696 (39–225 MHz). Hnα RRLs were detected in emission for n = 272–480 (59–325 MHz). Hnβ lines were detected for n = 387–453 (141–225 MHz). The stacked Hnα detection at 63 MHz is the lowest frequency detection made for hydrogen RRLs and shows that a cold (partially) ionized medium exists along the line of sight to the Galactic Centre region. The size and velocity of this cold H+ gas indicates that it is likely associated with the nearby Riegel–Crutcher cloud.