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

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Belfiore, F 
Tremonti, C 
Sanchez, SF 
Bundy, K 


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 Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory 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 > 1.5Re), but a flattening is observed in the central regions (R < 1Re). In the outer regions (R > 2.0Re), 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.



galaxies: evolution, galaxies: fundamental parameters, galaxies: ISM

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Monthly Notices of the Royal Astronomical Society

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Oxford University Press
European Research Council (695671)
FB and RM acknowledge funding from the United Kingdom Science and Technology Facilities Council (STFC). RM acknowledges funding from the European Research Council (ERC), Advanced Grant 695671 ‘QUENCH’. SFS thanks the CONACYT- 125180, DGAPA-IA100815 and DGAPA-IA101217 projects for providing him support in this study. CAT acknowledges support from National Science Foundation of the United States grant 1412287 and 1554877. This work makes use of data from SDSS-IV. Funding for SDSS has been provided by the Alfred P. Sloan Foundation and Participating Institutions. Additional funding towards SDSS-IV has been provided by the U.S. Department of Energy Office of Science. SDSS-IV acknowledges support and resources from the Centre for High-Performance Computing at the University of Utah.