THE LICK-INDEX CALIBRATION OF THE GEMINI MULTI-OBJECT SPECTROGRAPHS

We present the calibration of the spectroscopic Lick/IDS standard line-index system for measurements obtained with the Gemini Multi-Object Spectrographs known as GMOS-North and GMOS-South. We provide linear correction functions for each of the 25 standard Lick line indices for the B600 grism and two instrumental setups, one with 0".5 slit width and 1 x 1 CCD pixel binning (corresponding to similar to 2.5 angstrom spectral resolution) and the other with 0".15 slit width and 2 x 2 binning (similar to 4 angstrom). We find small and well-defined correction terms for the set of Balmer indices H beta, H gamma(A), and H delta(A) along with the metallicity sensitive indices Fe5015, Fe5270, Fe5335, Fe5406, Mg-2, and Mgb that are widely used for stellar population diagnostics of distant stellar systems. We find other indices that sample molecular absorption bands, such as TiO1 and TiO2, with very wide wavelength coverage or indices that sample very weak molecular and atomic absorption features, such as Mg-1, as well as indices with particularly narrow passband definitions, such as Fe4384, Ca4455, Fe4531, Ca4227, and Fe5782, which are less robustly calibrated. These indices should be used with caution.