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. Cn$\alpha$ RRLs were detected in absorption for quantum levels n
= 378 to 550 (39-121 MHz) and in emission for n = 272 to 306 (228-325 MHz).
Cn$\beta$ lines were detected in absorption for n = 387 to 696 (39-225 MHz).
Hn$\alpha$ RRLs were detected in emission for n = 272 to 480 (59-325 MHz).
Hn$\beta$ lines were detected for n = 387 to 453 (141-225 MHz). The stacked
Hn$\alpha$ 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.
