Low-frequency Radio Recombination Lines Away from the Inner Galactic Plane

Diffuse radio recombination lines (RRLs) in the Galaxy are possible foregrounds for redshifted 21 cm experiments. We use EDGES drift scans centered at -26 degrees.7 decl. to characterize diffuse RRLs across the southern sky. We find that RRLs averaged over the large antenna beam (72 degrees x 110 degrees) reach minimum amplitudes of R.A. = 2-6 hr. In this region, the C alpha absorption amplitude is 33 +/- 11 mK (1 sigma) averaged over 50-87 MHz (27 greater than or similar to z greater than or similar to 15 for the 21 cm line) and increases strongly as frequency decreases. C beta and H alpha lines are consistent with no detection with amplitudes of 13 +/- 14 and 12 +/- 10 mK (1 sigma), respectively. At 108-124.5 MHz (z approximate to 11) in the same region, we find no evidence for carbon or hydrogen lines at the noise level of 3.4 mK (1 sigma). Conservatively assuming that observed lines come broadly from the diffuse interstellar medium, as opposed to a few compact regions, these amplitudes provide upper limits on the intrinsic diffuse lines. The observations support expectations that Galactic RRLs can be neglected as significant foregrounds for a large region of sky until redshifted 21 cm experiments, particularly those targeting cosmic dawn, move beyond the detection phase. We fit models of the spectral dependence of the lines averaged over the large beam of EDGES, which may contain multiple line sources with possible line blending, and find that including degrees of freedom for expected smooth, frequency-dependent deviations from local thermodynamic equilibrium (LTE) is preferred over simple LTE assumptions for C alpha and H alpha lines. For C alpha we estimate departure coefficients 0.79 < b(n)beta(n) < 4.5 along the inner Galactic plane and 0 < b(n)beta(n) < 2.3 away from the inner Galactic plane.