Extragalactic globular clusters. I - The metallicity calibration

We have explored the ability of absorption-line strength indices, measured from integrated globular cluster spectra, to predict mean cluster metallicity. Using statistical criteria, we have identified the six best indices out of ~20 measured in a large sample of Galactic and M31 cluster spectra. Linear relations between index and metallicity have been derived along with new calibrations of infrared colors (V-K, J-K, and CO) versus [Fe/H]. Estimates of metallicity from the six spectroscopic index-metallicity relations have been combined in three different ways to identify the most efficient estimator and the minimum bias estimator of [Fe/H]--the weighted mean. This provides an estimate of [Fe/H] accurate to about 15%. Cyanogen at 4170 A correlates with metallicity, but there is a significant offset between Galactic and M31 clusters which may be due to an abundance anomaly. Reddenings for individual M31 clusters can be estimated to an accuracy of about 0.04 in E(B- V) by comparing metallicities derived from line-strength indices with those derived from infrared colors.