2025ApJ...995L..44Y

As a typical secondary element, nitrogen is produced mainly by hydrogen burning in intermediate-mass stars, while α-elements such as carbon and oxygen originate from helium burning in more massive stars. Nitrogen abundance Z_N and abundance ratio N/α serve as living fossil for investigating galaxy chemical evolution. A peculiar subclass of active galaxies, namely, “nitrogen-loud” active galactic nuclei (AGN), are of particular interest, with exceptionally strong [N III] λ1750 and [N IV] λ1486 broad emission lines (BELs), but then normal in all of the other BELs. Although it is generally believed that both Z_N and N/α are extremely high in the broad-line region (BLR), the nature of their host galaxies remains elusive after more than two decades of extensive studies, as all known nitrogen-loud AGN are far away at z > 1.6. Here we identify the prototype NLS1 Akn 564 (z = 0.024) as the first local nitrogen-loud AGN, with [N III] and [N IV] twice as strong as in the composite spectrum of nitrogen-loud AGN. Emission lines commonly used for abundance diagnostics are all detected with high signal-to-noise ratio. The α-elements show a significant difference between the galactic and the inner galactic region, increasing from Z_α,H II_(5.4 kpc) ≈ 0.3 Z_⊙ to Z_α,NLR(100 pc) ≈ Z_α,BLR(0.01 pc) ≈ 0.6-0.7 Z_⊙, while a steep gradient is found for N/α, jumping from (N/α)_NLR ≈ 1(N/α)_⊙ to (N/α)_BLR ≈ 4 (N/α)_⊙within Δr ≲ 100 pc. This finding indicates different star-formation histories at different scales and the AGN activity possibly driven by the stellar processes in the circumnuclear region, which could be investigated in detail by follow-up high spatial resolution 3D spectroscopy.