# Low-Frequency Radio Recombination Lines Away From the Inner Galactic Plane

## TL;DR

This study characterizes diffuse radio recombination lines across the southern sky, finding they are weak and unlikely to significantly interfere with 21cm cosmology experiments targeting the Cosmic Dawn.

## Contribution

It provides the first large-scale measurements and upper limits of diffuse RRLs away from the Galactic plane, informing foreground modeling for 21cm observations.

## Key findings

- RRL amplitudes are minimal between RA 2-6h.
- No significant carbon or hydrogen lines detected at 108-124.5 MHz.
- Galactic RRLs are negligible as foregrounds for most of the sky in 21cm experiments.

## Abstract

Diffuse radio recombination lines (RRLs) in the Galaxy are possible foregrounds for redshifted 21~cm experiments. We use EDGES drift scans centered at $-26.7^o$~declination to characterize diffuse RRLs across the southern sky. We find RRLs averaged over the large antenna beam ($ 72^o \times 110^o $) reach minimum amplitudes between right ascensions~2-6~h. In this region, the C$\alpha$ absorption amplitude is $33\pm11$~mK (1$\sigma$) averaged over 50-87~MHz ($27\gtrsim z \gtrsim15$ 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 $13\pm14$ and $12\pm10$~mK (1$\sigma$), respectively. At 108-124.5~MHz ($z\approx11$) 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 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.