NEW RADIAL ABUNDANCE GRADIENTS FOR NGC 628 AND NGC 2403

Abstract

Motived by recent ISM studies, we present high quality MMT and Gemini\nspectroscopic observations of H II regions in the nearby spiral galaxies NGC\n628 and NGC 2403 in order to measure their chemical abundance gradients. Using\nlong-slit and multi-object mask optical spectroscopy, we obtained measurements\nof the temperature sensitive auroral lines [O III] {\\lambda}4363 and/or [N II]\n{\\lambda}5755 at a strength of 4{\\sigma} or greater in 11 H II regions in NGC\n628 and 7 regions in NGC 2403. These observations allow us, for the first time,\nto derive an oxygen abundance gradient in NGC 628 based solely on "direct"\noxygen abundances of H II regions: 12 + log(O/H) = (8.43+/-0.03) +\n(-0.017+/-0.002) x Rg (dex/kpc), with a dispersion in log(O/H) of {\\sigma} =\n0.10 dex, from 14 regions with a radial coverage of ~2-19 kpc. This is a\nsignificantly shallower slope than found by previous "strong-line" abundance\nstudies. In NGC 2403, we derive an oxygen abundance gradient of 12 + log(O/H) =\n(8.48+/-0.04) + (-0.032+/-0.007) x Rg (dex/kpc), with a dispersion in log(O/H)\nof {\\sigma} = 0.07 dex, from 7 H II with a radial coverage of ~1-10 kpc.\n Additionally, we measure the N, S, Ne, and Ar abundances. We find the N/O\nratio decreases with increasing radius for the inner disk, but reaches a\nplateau past R25 in NGC 628. NGC 2403 also has a negative N/O gradient with\nradius, but we do not sample the outer disk of the galaxy past R25 and so do\nnot see evidence for a plateau. This bi-modal pattern measured for NGC 628\nindicates dominant contributions from secondary nitrogen inside of the R25\ntransition and dominantly primary nitrogen farther out. As expected for\n{\\alpha}-process elements, S/O, Ne/O, and Ar/O are consistent with constant\nvalues over a range in oxygen abundance.\n