Cleaning spectroscopic samples of stars in nearby dwarf galaxies

Abstract

Dwarf galaxies provide insight into the processes of star formation and chemical enrichment at the low end of the galaxy mass function, as well as into 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, which are not related to the dwarf galaxy, since they can contaminate analysis of the properties of these objects. Here we introduce a new diagnostic for separating Milky Way contaminant stars, which mainly consist 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 Mg I line at 8806.8 Å as a function of the equivalent width of Ca II triplet lines. This method is particularly useful for works dealing with multi-object, intermediate-resolution spectroscopy focusing in the region of the nIR Ca II 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 distinction among giants above the horizontal branch and dwarfs can be made with this method at [Fe/H] > −2 dex. For −2 ≤ [Fe/H] ≤ −1, this method is also valid for distinguishing 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 dwarf spheroidal galaxies to verify the predicted theoretical trends.

The extremely low-metallicity tail of the Sculptor dwarf spheroidal galaxy

“…Following Battaglia et al (2008a) we use a systemic velocity v hel,sys = 110.6 km s −1 and a velocity dispersion σ = 10.1 km s −1 . Due to the high latitude and distinct radial velocity of Sculptor from the Milky Way disk, the chance to accidentally select interlopers with this radial velocity criterion is very small (Battaglia & Starkenburg 2012). Figure 1 shows the equivalent widths and absolute magnitude of the selected targets along with the calibration of Starkenburg et al (2010) which deviates from earlier linear calibrations for very metal-poor stars.…”

VLT/FLAMES spectroscopy of red giant branch stars in the Fornax dwarf spheroidal galaxy

“…Therefore, they are very probably foreground stars (We checked that mem0598 does not seem to be a CH star). On the other hand, mem0598 (the only star for which we have spectra in the CaT region) is not considered as dwarf foreground contaminant according to the λ = 8806.8 Å MgI line criterion (Battaglia & Starkenburg 2012).…”

“…As a result, our sample appears slightly biased as it misses (in relative numbers) RGB stars between -2.0 and -1.0 dex. The CaT sample might also incorporate some foreground stars that match the velocity range for Fornax membership, but their number should be quite low: combining the Mg I line at λ = 8806.8 Å and the CaT, Battaglia & Starkenburg (2012) can remove the MW dwarf stars contaminating the samples of RGB kinematic members in the dSphs. They estimate the level of contamination to be 5%.…”

Orbit-based dynamical models of the Sculptor dSph galaxy

“…We add this requirement since we are not confident that outside this radius a reliable velocity dispersion can be measured due to the low number density of (probable) Sculptor members compared to Milky Way stars. An improved method for discriminating Milky Way contaminants based on surface gravity in the data set of B08 has been developed by (Battaglia & Starkenburg 2012), see also .…”