The M33 metallicity project: Resolving the abundance gradient discrepancies in M33

Creators: Rosolowsky, Erik; Simon, Joshua D.

Abstract

We present a new determination of the metallicity gradient in M33, based on Keck LRIS measurements of oxygen abundances using the temperature-sensitive emission line [O III] λ4363 in 61 H II regions. These data approximately triple the sample of direct oxygen abundances in M33. We find a central abundance of 12 + log (O/H) = 8.36 ± 0.04 and a slope of –0.027 ± 0.012 dex kpc^−1, in agreement with infrared measurements of the neon abundance gradient but much shallower than most previous oxygen gradient measurements. There is substantial intrinsic scatter of 0.11 dex in the metallicity at any given radius in M33, which imposes a fundamental limit on the accuracy of gradient measurements that rely on small samples of objects. We also show that the ionization state of neon does not follow the ionization state of oxygen as is commonly assumed, suggesting that neon abundance measurements from optical emission lines require careful treatment of the ionization corrections.

Additional Information

© 2008 The American Astronomical Society. Received 2007 April 12; accepted 2007 November 27. Print publication: Issue 2 (2008 March 10). The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We thank the anonymous referee for comments that improved the paper. E. R. acknowledges support from an NSF AAP Fellowship (AST 05-02605), and J. D. S. acknowledges the support of a Millikan Fellowship provided by Caltech. We thank Leo Blitz for support of the early stages of this project, Joe Hennawi for assistance with our data reduction code, and Jose Vılchez for helpful conversations. We also acknowledge the many useful discussions with the participants in the Metals 2007 Conference whose advice has improved this work. This research has made use of NASA's Astrophysics Data System Bibliographic Services and the SIMBAD database, operated at CDS, Strasbourg, France. Facilities: Keck:I (LRIS). Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

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