Cleaning spectroscopic samples of stars in nearby dwarf galaxies: The use of the nIR MgI line to weed out Milky Way contaminants

TL;DR

This paper introduces a new spectroscopic diagnostic using the nIR MgI line to effectively distinguish Milky Way dwarf star contaminants from red giant stars in dwarf galaxy samples, improving the accuracy of galaxy property analyses.

Contribution

The study presents a novel method based on the MgI line equivalent width to separate dwarf and giant stars, validated with synthetic spectra and VLT/FLAMES data for multiple dwarf spheroidal galaxies.

Findings

Effective discrimination for [Fe/H]> -2 dex.

Valid for stars up to one magnitude below the horizontal branch.

Method confirmed with observational data from Sextans, Sculptor, and Fornax.

Abstract

Dwarf galaxies provide insights on the processes of star formation and chemical enrichment at the low end of the galaxy mass function, as well as on the clustering of dark matter on small scales. In studies of Local Group dwarf galaxies, spectroscopic samples of individual stars are used to derive the internal kinematics and abundance properties of these galaxies. It is therefore important to clean these samples from Milky Way stars, not related to the dwarf galaxy, since they can contaminate the analysis of the properties of these objects. Here we introduce a new diagnostic for separating Milky Way contaminant stars -- that mainly constitute of dwarf stars -- and red giant branch stars targeted in dwarf galaxies. As discriminator we use the trends in the equivalent width of the nIR MgI line at 8806.8 \AA\ as a function of the equivalent width of CaII triplet lines. This method is particularly useful for works dealing with multi-object intermediate resolution spectroscopy focusing in the region of the nIR CaII triplet. We use synthetic spectra to explore how the equivalent width of these lines changes for stars with different properties (gravity, effective temperature, metallicity) and find that a discrimination among giants above the horizontal branch and dwarfs can be made with this method at [Fe/H]> -2 dex. For -2 $\le$ [Fe/H] $\le$ -1, this method is also valid to discriminate dwarfs and giants down to approximately one magnitude below the horizontal branch. Using a foreground model we make predictions on the use of this new discrimination method for nearby dwarf spheroidal galaxies, including the ultra-faints. We subsequently use VLT/FLAMES data for the Sextans, Sculptor and Fornax dSphs to verify the predicted theoretical trends.