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For refcode 1999ApJ...514..544K:
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1999ApJ...514..544K On Measuring Nebular Chemical Abundances in Distant Galaxies Using Global Emission-Line Spectra HENRY A. KOBULNICKY University of California, Santa Cruz Lick Observatory/Board of Studies in Astronomy, Santa Cruz, CA 95064; chip@ucolick.org AND ROBERT C. KENNICUTT, JR. AND JAMES L. PIZAGNO University of Arizona, Steward Observatory, Tucson, AZ 85721; robk@as.arizona.edu, jpizagno@as.arizona.edu Received 1998 August 25; accepted 1998 November 5 ABSTRACT The advent of 8-10 m class telescopes enables direct measurement of the chemical properties in the ionized gas of cosmologically distant galaxies with the same nebular analysis techniques used in local H II regions. We show that spatially unresolved (i.e., global) emission-line spectra can reliably indicate the chemical properties of distant star-forming galaxies. However, standard nebular chemical abundance measurement methods (those with a measured electron temperature from [O III] {lambda}4363) may be subject to small systematic errors when the observed volume includes a mixture of gas with diverse temperatures, ionization parameters, and metallicities. To characterize these systematic effects, we compare physical conditions derived from spectroscopy of individual H II regions with results from global galaxy spectroscopy. We consider both low-mass, metal-poor galaxies with uniform abundances and larger galaxies with internal chemical gradients. For low-mass galaxies, standard chemical analyses using global spectra produce small systematic errors in that the derived electron temperatures are 1000-3000 K too high due to nonuniform electron temperatures and large variations in the ionization parameter. As a result, the oxygen abundances derived from direct measurements of the electron temperatures are too low, but it is possible to compensate for this effect by applying a correction of {DELTA}(O/H)<= +0.1 dex to the oxygen abundances derived from global spectra. For more massive metal-rich galaxies like local spiral galaxies, direct measurements of electron temperatures are seldom possible from global spectra. Well-established empirical calibrations using strong-line ratios can serve as reliable (10.2 dex) indicators of the overall systemic oxygen abundance even when the signal to noise of the H[β] and [O III] emission lines is as low as 8:1. We present prescriptions, directed toward high-redshift observers, for using global emission-line spectra to trace the chemical properties of star-forming galaxies in the distant universe. Subject headings: cosmology: observations-galaxies: abundances -galaxies: ISM-galaxies: spiral-H II regions -ISM: abundances-techniques: spectroscopic
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