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

TL;DR

This paper develops and tests a method for deriving detailed chemical abundances from co-added medium-resolution spectra of red giant stars, enabling studies of distant galaxies where individual spectra are too faint.

Contribution

It introduces a spectral co-addition technique for measuring metallicities and alpha-element abundances, validated with Milky Way dwarf galaxies and globular clusters, to extend detailed abundance analysis to more distant systems.

Findings

High agreement between co-added spectra and individual star measurements

Method enables abundance studies of distant galaxies like M31

Potential biases in spectral co-addition are identified and discussed

Abstract

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