Calibrating Multi-Wavelength Metallicity Diagnostics for Star-Forming Galaxies
We propose to obtain low-resolution COS-FUV spectra of a sample of nearby star-forming galaxies with existing rest-frame optical spectroscopy. By combining optical and UV observations of these galaxies, we will be able to directly compare previously-determined metallicities from optical emission line diagnostics with those determined from newly-observed photospheric and stellar wind lines in the UV, including the abundance-sensitive 1425A and 1978A photospheric blends. While each of these metallicity diagnostics has been individually examined and calibrated, there is currently no quantitative conversion relating diagnostics between the optical and UV regimes. These observations will offer the first empirical sample that can be used, in conjunction with stellar population synthesis and photoionization models, to develop a self-consistent multi-wavelength baseline for unifying metallicity measurements in star-forming galaxies spanning a broad range of redshifts. In addition, rest-frame UV galaxy spectra such as these will be exceptionally important in the coming decade as the Extremely Large Telescopes come online. At the higher redshifts made reachable from the ground by these facilities, the UV regime will become the "new" rest-frame for ground-based extragalactic astronomy, demanding a thorough understanding of UV spectra and diagnostics.
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