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تسجيل مستخدم جديدSDSS IV MaNGA - Metallicity and nitrogen abundance gradients in local galaxies
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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.
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