SDSS IV MaNGA – metallicity and nitrogen abundance gradients in local galaxies

We study the gas phase metallicity (O/H) and nitrogen abundance gradients traced by star forming regions in a representative sample of 550 nearby galaxies in the stellar mass range 10⁹-1011.5 M_ with resolved spectroscopic data from the SDSS-IV MaNGA survey. Using strong-line ratio diagnostics (R23 and O3N2 for metallicity and N2O2 for N/O) and referencing to the effective (half-light) radius (Rₑ), we find that the metallicity gradient steepens with stellar mass, lying roughly flat among galaxies with log(M_/M₎ = 9.0 but exhibiting slopes as steep as -0.14 dex Rₑ⁻¹ at log(M_/M₎ = 10.5 (using R23, but equivalent results are obtained using O3N2). At higher masses, these slopes remain typical in the outer regions of our sample (R > 1.5 ~Rₑ), but a flattening is observed in the central regions (R < 1~ Rₑ). In the outer regions (R > 2.0 ~Rₑ) we detect a mild flattening of the metallicity gradient in stacked profiles, although with low significance. The N/O ratio gradient provides complementary constraints on the average chemical enrichment history. Unlike the oxygen abundance, the average N/O profiles do not flatten out in the central regions of massive galaxies. The metallicity and N/O profiles both depart significantly from an exponential form, suggesting a disconnect between chemical enrichment and stellar mass surface density on local scales. In the context of inside-out growth of discs, our findings suggest that central regions of massive galaxies today have evolved to an equilibrium metallicity, while the nitrogen abundance continues to increase as a consequence of delayed secondary nucleosynthetic production.