New radial abundance gradients for NGC 628 and NGC 2403

Danielle A. Berg, Evan D. Skillman, Donald R. Garnett, Kevin V. Croxall, Andrew R. Marble, J. D. Smith, Karl Gordon, Robert C. Kennicutt

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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) × Rg (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)× Rg (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.

Original languageEnglish (US)
Article number128
JournalAstrophysical Journal
Issue number2
StatePublished - Oct 1 2013


  • galaxies: abundances
  • galaxies: evolution
  • galaxies: individual (NGC 628, NGC 2403)
  • galaxies: spiral


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