Radio Recombination Lines at Decametre Wavelengths: Prospects for the
  Future
  This paper considers the suitability of a number of emerging and future
instruments for the study of radio recombination lines (RRLs) at frequencies
below 200 MHz. These lines arise only in low-density regions of the ionized
interstellar medium, and they may represent a frequency-dependent foreground
for next-generation experiments trying to detect H I signals from the Epoch of
Reionization and Dark Ages ("21-cm cosmology"). We summarize existing
decametre-wavelength observations of RRLs, which have detected only carbon
RRLs. We then show that, for an interferometric array, the primary instrumental
factor limiting detection and study of the RRLs is the areal filling factor of
the array. We consider the Long Wavelength Array (LWA-1), the LOw Frequency
ARray (LOFAR), the low-frequency component of the Square Kilometre Array
(SKA-lo), and a future Lunar Radio Array (LRA), all of which will operate at
decametre wavelengths. These arrays offer digital signal processing, which
should produce more stable and better defined spectral bandpasses; larger
frequency tuning ranges; and better angular resolution than that of the
previous generation of instruments that have been used in the past for RRL
observations. Detecting Galactic carbon RRLs, with optical depths at the level
of 10^-3, appears feasible for all of these arrays, with integration times of
no more than 100 hr. The SKA-lo and LRA, and the LWA-1 and LOFAR at the lowest
frequencies, should have a high enough filling factor to detect lines with much
lower optical depths, of order 10^-4 in a few hundred hours. The amount of
RRL-hosting gas present in the Galaxy at the high Galactic latitudes likely to
be targeted in 21-cm cosmology studies is currently unknown. If present,
however, the spectral fluctuations from RRLs could be comparable to or exceed
the anticipated H I signals.
