On the reliability of C IV λ1549 as an abundance indicator for high-redshift star-forming galaxies

We reconsider the use of the equivalent width of C IV lambda 1549, EW( C IV), as an indicator of the oxygen abundance in star-forming galaxies, as proposed by Heckman et al. for nearby starbursts. We refine the local calibration of EW( C IV) versus log (O/H) by using a restricted wavelength window which minimizes blending with interstellar absorption lines. When applied to the stellar component only of the complex CIV zeta 1549 features in two high-redshift galaxies with good quality spectra, MS 1512-cB58 ( z = 2.7268) and Q1307-BM1163 ( z = 1.4105), the local calibration gives values of the oxygen abundance which are in good agreement with other metallicity determinations based on nebular emission and interstellar absorption lines. Our main conclusion is that for this method to give reliable results at high redshifts, it should only be used on data of sufficiently high spectral resolution ( R greater than or similar to 1000) for stellar and interstellar CIV components to be clearly separated. Oxygen abundances will be systematically overestimated if the local calibration is applied to spectra of high-redshift galaxies obtained with the low resolving powers ( R similar or equal to 200 - 300) of many current wide-field surveys. It will also be necessary to understand better the causes of the scatter in the local relation, before we can be confident of inferences from it at high z.