2025AJ....170..276R

We present the first detection of helium radio recombination lines (HeRRLs) at 800 MHz from the diffuse ionized gas (DIG) in the Cygnus X region, with an ionized helium-to-hydrogen ratio (${{n}}_{{\rm{H}}{\rm{e}}}^{+}/{{n}}_{{\rm{H}}}^{+}$) of 0.06 ± 0.02, consistent with values observed in Galactic H II regions ionized by O6 stars or earlier. The data were obtained from a multifrequency RRL survey (342, 800, and 5800 MHz) using the Robert C. Byrd Green Bank Telescope, toward a region centered at $l=80\mathop{.}\limits^{^\circ }0$, $b=1\mathop{.}\limits^{^\circ }4$, with an extent of $\pm 2\mathop{.}\limits^{^\circ }85$. Several techniques were applied to mask out RRL emission from H II regions, and the remaining data were averaged to achieve the sensitivity required to detect HeRRLs from the DIG. Analysis of the pixel-to-pixel hydrogen line central velocities suggests that the 342 and 800 MHz line-forming regions are associated. Modeling of the averaged data indicates that the emission at these frequencies is dominated by an ionized component with an electron density of 2-15 cm^–3 and an EM of 750-1600 pc cm^–6 for an electron temperature of 8000 K. The derived properties of the DIG in Cygnus X are consistent with those reported for diffuse gas in the inner Galaxy. Previous attempts to detect HeRRLs in the inner Galaxy only yielded upper limits of 0.013, much lower than the value found in Cygnus X. Our detection has significant implications, including the potential for an additional hydrogen ionization source and changes in dust properties in the inner Galaxy.