Pinning Down the Chemical Abundances of Metal-Rich Giant HII Regions

Details Pinning Down the Chemical Abundances of Metal-Rich Giant HII Regions Pinning Down the Chemical Abundances of Metal-Rich Giant HII Regions

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006sptz.prop30205b&link_type=abstract

Spitzer Proposal ID #30205

Physics

Scientific paper

Measuring chemical abundances for giant extragalactic HII regions is crucial to understand the evolution of star-forming galaxies. Yet, there are lingering uncertainties regarding the techniques used for abundance determination at the high end of the metallicity scale (roughly solar and above). The so-called 'statistical' methods, used commonly for measuring abundances in nearby spirals as well as in high-redshift star-forming galaxies, provide oxygen abundances that are up to 2-3 times larger than those obtained from 'direct' methods in the case of metal-rich nebulae. This discrepancy affects a variety of studies on the nature of gaseous nebulae. In the past few years our group has collected Keck and VLT optical spectra of metal-rich HII regions in spiral galaxies with the purpose of deriving direct abundances, modeling their ionization and thermal structures, and study their massive stellar content. Out of this sample we have selected 10 HII regions located in five galaxies and that are believed, based on direct methods, to have oxygen abundances betweeen 0.7 and 1.7 times the solar value. We are proposing to obtain Spitzer IRS Short-High spectra for these 10 giant HII regions in order to measure the following emission lines: [Ne II] 12.8, [Ne III] 15.6, [S III] 18.7 and [S IV] 10.5. These fine-structure lines provide sensitive probes of metal abundances. Forming ratios between these lines and the available optical lines offers powerful diagnostics of the hardness of the stellar ionizing field and of the nebular temperature. By fully exploiting the combination of optical and infrared spectra it will be possible to verify the presence of severe abundance biases at high metallicity, as predicted for metal-rich HII regions by 'ab initio' models, and eventually to obtain significantly improved determinations of chemical abundances in giant HII regions, and in star-forming galaxies in the Universe in general.

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