Analysis and calibration of CaII triplet spectroscopy of Red Giant Branch stars from VLT/FLAMES observations

We demonstrate that low resolution CaII triplet (CaT) spectroscopic estimates of the overall metallicity ([Fe/H]) of individual Red Giant Branch (RGB) stars in two nearby dwarf spheroidal galaxies (dSphs) agree to +-0.1-0.2 dex with detailed high resolution spectroscopic determinations for the same stars over the range -2.5 < [Fe/H] < -0.5. For this study we used a sample of 129 stars observed in low and high resolution mode with VLT/FLAMES in the Sculptor and Fornax dSphs. We also present the data reduction steps we used in our low resolution analysis and show that the typical accuracy of our velocity and CaT [Fe/H] measurement is ~2 km/s and 0.1 dex respectively. We conclude that CaT-[Fe/H] relations calibrated on globular clusters can be applied with confidence to RGB stars in composite stellar populations over the range -2.5 < [Fe/H] < -0.5.