Nitrogen Abundance Distribution in the Inner Milky Way

We combine a new Galactic plane survey of Hydrogen Radio Recombination Lines (RRLs) with surveys of the far–infrared (FIR) lines of ionized nitrogen, N + , to derive the nitrogen abundance as a function of Galactic radius for comparison to theories of the chemical evolution of galaxies. The RRLs were observed with the NASA DSS–43 70m antenna and the Green Bank Telescope in 108 lines–of– sight covering the range −135 ◦ < l < 60 ◦ in Galactic longitude, at a latitude of b = 0 ◦ . These positions have also been observed in the [N ii] 122μm and 205μm, and [C ii] 158μm lines with the Herschel Space Observatory. RRLs are tracers of ionized gas and they can be used to study the dynamics and properties of the ionized component of the interstellar medium. We combined RRL and [N ii] 122μm and 205μm observations in 41 of the 108 samples in the Galactic plane, for which we have high signaltonoise ratio, to study the distribution of the ionized nitrogen abundance relative to ionized hydrogen covering Galactocentric distances between 0 and 8 kpc. Combined with existing determinations of the N/H abundance in the solar neighborhood and outer galaxy, we are able to study for the first time the distribution of this quantity in the inner 16 kpc of the Milky Way. We find a nitrogen abundance gradient that extends from Galactocentric radii between 4 and 16 kpc in the Galactic plane, while for 0 to 4 kpc the N/H distribution is flat. Flattening of the O/H abundance ratio is observed in massive nearby galaxies, and a lack of flattening in the N/O ratio observed in these objects is interpreted as the N/H abundance continuing to increase toward the center of galaxies, contradicting our Milky Way results. The gradient we find at large Galactocentric distances is consistent with inside–out galaxy growth, while the flattening in the inner 4 kpc, which coincides with the start of the Galactic bar, can be associated with radial flows induced by the bar potential or that a nitrogen abundance equilibrium, in which metal production is balanced by metal consumption by star formation expulsion by outflows, has been reached as predicted by chemical evolution models. We used SOFIA/FIFI–LS and Herschel/PACS to observe the [N iii] 57μm line to trace the contribution of doubly ionized gas in a sub sample of sightlines. We find that along these sightlines the contribution of N ++ and higher ionization is small, such that most of the nitrogen in our sample is singly ionized, which is consistent with them originating from the low ionization outskirts of H ii regions.