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

G. Battaglia, Else Starkenburg|University of Groningen research database (University of Groningen / Centre for Information Technology)|Jan 17, 2012

Stellar, planetary, and galactic studiesPhysics and Astronomy49 references22 citations

TL;DR

This paper proposes using the near-infrared Mg I line at 8806.8 Å in combination with the Ca II triplet (CaT) line strength to distinguish red giant branch (RGB) stars in nearby dwarf galaxies from Milky Way dwarf star contaminants in spectroscopic samples. The method exploits differences in surface gravity between giants and dwarfs, enabling effective separation for metallicities above [Fe/H] = -2 dex, with validation on Sextans, Sculptor, and Fornax dSphs using VLT/FLAMES data.

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 Å 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.

Motivation & Objective

• To address the challenge of Milky Way foreground dwarf stars contaminating spectroscopic samples of red giant branch stars in nearby dwarf galaxies.
• To improve the accuracy of kinematic and abundance analyses in dwarf galaxy studies by reducing contamination from interlopers.
• To develop a physically motivated, gravity-sensitive diagnostic using spectroscopic lines that can be applied to existing intermediate-resolution data.
• To validate the method on real data from classical dSphs (Sextans, Sculptor, Fornax) and extend its applicability to ultra-faint dwarfs.
• To provide a simple, implementable criterion that complements velocity-based selection and enhances sample purity, especially in outer regions and low-surface-brightness systems.

Proposed method

• The method uses the equivalent width (EW) of the near-infrared Mg I line at 8806.8 Å as a gravity-sensitive discriminator, combined with the Ca II triplet (CaT) line strength (ΣW).
• Synthetic spectra are generated across a grid of effective temperature, surface gravity, and metallicity to model how Mg I and CaT EWs vary for giants and dwarfs.
• The relationship between Mg I EW and CaT ΣW is calibrated to separate giants (dwarf galaxy members) from dwarfs (Milky Way contaminants) based on gravity differences.
• The method is tested on VLT/FLAMES data from three classical dSphs—Sextans, Sculptor, and Fornax—where the theoretical trends are confirmed by observed data.
• The approach is applied to predict performance in ultra-faint dwarfs and other dSphs, with emphasis on systems where velocity-based selection is less effective.
• The method is designed to be easily implemented on existing data, requiring only measurement of two strong, closely spaced lines in the near-infrared region.

Experimental results

Research questions

• RQ1 Can the Mg I line at 8806.8 Å be used as a reliable gravity indicator to distinguish between red giant branch stars and Milky Way dwarf contaminants in spectroscopic samples?
• RQ2 How effective is the combination of Mg I EW and CaT ΣW in separating dwarfs from giants across different metallicities, particularly in the range [Fe/H] > -2 dex?
• RQ3 To what extent does this method improve contamination rejection compared to velocity-based selection alone, especially in low-surface-brightness or outer halo regions?
• RQ4 How well do synthetic models of Mg I and CaT lines reproduce observed trends in real dSph data, such as in Sextans, Sculptor, and Fornax?
• RQ5 In which dwarf galaxy systems—particularly ultra-faint dwarfs—is this method most beneficial due to limitations in velocity-based selection?

Key findings

• The Mg I EW–CaT ΣW relation successfully separates giants from dwarfs for metallicities above [Fe/H] = -2 dex, where the majority of contaminants reside.
• For metallicities between -2 and -1 dex, the method also effectively distinguishes dwarfs from giants down to approximately one magnitude below the horizontal branch.
• The method is validated on real VLT/FLAMES data from the classical dSphs Sextans, Sculptor, and Fornax, where theoretical trends match observed data well.
• The method is particularly effective in systems like Canes Venetici I, Leo II, Fornax, Hercules, Ursa Major I, and Willman 1, where velocity-based selection alone is insufficient.
• For ultra-faint dwarfs and systems with high foreground contamination, the Mg I method can be more efficient than radial velocity selection alone.
• The method does not remove Milky Way giant contaminants, but it effectively removes the dominant contaminant class—dwarf stars—while preserving the target giant population.