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The ability to measure metallicities and {\alpha}-element abundances inindividual red giant branch (RGB) stars using medium-resolution spectra ($R\approx 6000$) is a valuable tool for deciphering the nature of Milky Way dwarfsatellites and the history of the Galactic halo. Extending such studies to moredistant systems like Andromeda is beyond the ability of the current generationof telescopes, but by co-adding the spectra of similar stars, we can attain thenecessary signal-to-noise ratio to make detailed abundance measurements. Inthis paper, we present a method to determine metallicities and {\alpha}-elementabundances using the co-addition of medium resolution spectra. We test themethod of spectral co-addition using high-S/N spectra of more than 1300 RGBstars from Milky Way globular clusters and dwarf spheroidal galaxies obtainedwith the Keck II telescope/DEIMOS spectrograph. We group similar stars usingphotometric criteria and compare the weighted ensemble average abundances([Fe/H], [Mg/Fe], [Si/Fe], [Ca/Fe] and [Ti/Fe]) of individual stars in eachgroup with the measurements made on the corresponding co-added spectrum. Wefind a high level of agreement between the two methods, which permits us toapply this co-added spectra technique to more distant RGB stars, like stars inthe M31 satellite galaxies. This paper outlines our spectral co-addition andabundance measurement methodology and describes the potential biases in makingthese measurements.

astrophysical journal/the astrophysical journal

Measuring Detailed Chemical Abundances From Co-added Medium Resolution  Spectra I. Tests using Milky Way dwarf spheroidal galaxies and globular  clusters