2024ApJ...973...89P

We combine a new Galactic plane survey of hydrogen radio recombination lines (RRLs) with far-infrared surveys of ionized nitrogen, N⁺, to determine nitrogen abundance across Galactic radius. RRLs were observed with the NASA Deep Space Network Station 43 70 m antenna and the Green Bank Telescope in 108 lines of sight spanning -135°< l < 60°, at b = 0°. These positions were also observed in [N II] 122 μm and 205 μm lines with the Herschel Space Observatory. Combining RRL and [N II] 122 μm and 205 μm observations in 41 of 108 samples with high signal-to-noise ratio, we studied the ionized nitrogen abundance distribution across Galactocentric distances of 0-8 kpc. Combined with existing solar neighborhood and outer Galaxy N/H abundance determinations, we studied this quantity's distribution within the Milky Way's inner 17 kpc for the first time. We found a nitrogen abundance gradient extending from Galactocentric radii of 4-17 kpc in the Galactic plane, while within 0-4 kpc the N/H distribution remained flat. The gradient observed at large Galactocentric distances supports inside-out galaxy growth, with the additional steepening resulting from variable star formation efficiency and/or radial flows in the Galactic disk, while the inner 4 kpc flattening, coinciding with the Galactic bar's onset, may be linked to radial flows induced by the bar potential. Using SOFIA/FIFI-LS and Herschel/PACS, we observed the [N III] 57 μm line to trace doubly ionized gas contribution in a subsample of sight lines. We found negligible N⁺⁺ contributions along these sight lines, suggesting mostly singly ionized nitrogen originating from low-ionization H II region outskirts.