The “Direct” Radial Abundance Gradients of NGC 628 and NGC 2403
Abstract
We present high quality spectroscopic observations of H II regions in two nearby spiral galaxies in order to measure their chemical abundance gradients. Using long-slit and multi-object mask optical spectroscopy from the MMT and Gemini, we obtained measurements of the temperature sensitive [O III] λ4363 and/or [N II] λ5755 auroral lines in H II regions selected for thorough radial coverage. Spiral galaxies pose a challenge to abundance work, as a single abundance measurement is not sufficient to characterize the entire galaxy. Therefore, high quality spectra of many H II regions that enable “direct” abundances are required to securely measure the oxygen abundance gradient. We present the first ever “direct” oxygen abundances for H II regions in NGC 628. From 13 regions with a radial coverage of 2.33 to 22.75 kpc we derive an oxygen abundance gradient of 12 + log(O/H) = 8.382 - 0.014 × Rg (dex/kpc), with a dispersion in log(O/H) of σ = 0.106. Similarly, for NGC 2403 we present an improved sample of “direct” oxygen abundances. From 7 H II regions extending from a galactocentric radius of 0.87 to 9.4 kpc we derive an oxygen abundance gradient of 12 + log(O/H) = 8.446-0.022×Rg (dex/kpc). Since there is a general paucity of high quality abundance data from individual spiral galaxies, these accurate H II region data sets are necessary to understand relative trends among galaxies.
- Publication:
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American Astronomical Society Meeting Abstracts #221
- Pub Date:
- January 2013
- Bibcode:
- 2013AAS...22114626B