Giuseppina Battaglia, Else Starkenburg
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.
View original:
http://arxiv.org/abs/1201.3634
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