New radial abundance gradients for NGC 628 and NGC 2403

Motivated by recent interstellar medium studies, we present high quality MMT and Gemini spectroscopic observations of H II regions in the nearby spiral galaxies NGC 628 and NGC 2403 in order to measure their chemical abundance gradients. Using long-slit and multi-object mask optical spectroscopy, we obtained measurements of the temperature sensitive auroral lines [O III] λ4363 and/or [N II] λ5755 at a strength of 4σ or greater in 11 H II regions in NGC 628 and 7 regions in NGC 2403. These observations allow us, for the first time, to derive an oxygen abundance gradient in NGC 628 based solely on "direct" oxygen abundances of H II regions: 12 + log(O/H) = (8.43 ± 0.03) + (-0.017 ± 0.002) × R_g (dex kpc^-1), with a dispersion in log(O/H) of σ = 0.10 dex, from 14 regions with a radial coverage of 2-19 kpc. This is a significantly shallower slope than found by previous "strong-line" abundance studies. In NGC 2403, we derive an oxygen abundance gradient of 12 + log(O/H) = (8.48 ± 0.04) + (-0.032 ± 0.007)× R_g (dex kpc^-1), with a dispersion in log(O/H) of σ = 0.07 dex, from seven H II with a radial coverage of 1-10 kpc. Additionally, we measure the N, S, Ne, and Ar abundances. We find the N/O ratio decreases with increasing radius for the inner disk, but reaches a plateau past R 25 in NGC 628. NGC 2403 also has a negative N/O gradient with radius, but we do not sample the outer disk of the galaxy past R 25 and so do not see evidence for a plateau. This bi-modal pattern measured for NGC 628 indicates dominant contributions from secondary nitrogen inside of the R 25 transition and dominantly primary nitrogen farther out. As expected for α-process elements, S/O, Ne/O, and Ar/O are consistent with constant values over a range in oxygen abundance.