SDSS IV MaNGA - Metallicity and nitrogen abundance gradients in local galaxies

Abstract

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

accepted in MNRAS, no change from previous version