On Measuring Nebular Chemical Abundances in Distant Galaxies Using Global Emission‐Line Spectra

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] λ4363) 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 Δ(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 (±0.2 dex) indicators of the overall systemic oxygen abundance even when the signal to noise of the Hβ 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.