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 $\rm 10^9-10^{11.5} M_\odot$ 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 ($\rm R_e$), we find that the metallicity
gradient steepens with stellar mass, lying roughly flat among galaxies with
$\rm log(M_\star/M_\odot) = 9.0$ but exhibiting slopes as steep as -0.14 dex
$\rm R_e^{-1}$ at $\rm log(M_\star/M_\odot) = 10.5$ (using R23, but equivalent
results are obtained using O3N2). At higher masses, these slopes remain typical
in the outer regions of our sample ($\rm R > 1.5 ~R_e$), but a flattening is
observed in the central regions ($\rm R < 1~ R_e$). In the outer regions ($\rm
R > 2.0 ~R_e$) 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.
