Nitrogen Abundance Distribution in the inner Milky Way

• Main Authors: Pineda, Jorge L, Horiuchi, Shinji, Anderson, L. D, Luisi, Matteo, Langer, William D, Goldsmith, Paul F, Kuiper, Thomas B. H, Fischer, Christian, Gong, Yan, Brunthaler, Andreas, Rugel, Michael, Menten, Karl M
• Format: Journal Article
• Language: English
• Published: 19-07-2024
• Subjects: Physics - Astrophysics of Galaxies
• DOI: 10.48550/arxiv.2407.12919

We combine a new Galactic plane survey of Hydrogen Radio Recombination Lines
(RRLs) with far-infrared (FIR) surveys of ionized Nitrogen, N+, to determine
Nitrogen abundance across Galactic radius. RRLs were observed with NASA DSS-43
70m antenna and the Green Bank Telescope in 108 lines-of-sight spanning -135
degrees < l < 60 degrees, at b=0 degrees. These positions were also observed in
[N II] 122 um and 205 um lines with the Herschel Space Observatory. Combining
RRL and [N II] 122 um and 205 um observations in 41 of 108 samples with high
signal-to-noise ratio, we studied 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 um line to trace doubly ionized gas
contribution in a sub-sample of sightlines. We found negligible N++
contributions along these sightlines, suggesting mostly singly ionized Nitrogen
originating from low ionization H II region outskirts.