Confirming membership in Local Group galaxies with the Dark Energy Spectroscopic Instrument Data Release 1

We use the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1) to identify stellar members of the Local Group dwarf galaxies. We cross-match DESI targets with candidate members that are based on Gaia proper motions, positions, and photometry. The addition of DESI radial velocities enables secure membership determination in 16 systems, spanning ultra-faint, classical, and irregular dwarf galaxies. Our results confirm that Gaia-based selection algorithms are effective in minimising foreground contamination. With DR1, we find the largest number of member stars for Draco and Sextans. While Draco is discussed in detail in other works, we focus on Sextans and, for the first time with DESI, trace its stellar kinematics to large radii (up to $\sim$10half-light radii). Our derived systemic velocity and velocity dispersion are consistent with previous works. We find that the more metal-poor population exhibits a higher velocity dispersion and extends to larger radii, whereas the more metal-rich population is kinematically colder and centrally concentrated. The metallicity gradient is steeper in the inner regions of Sextans ($\sim -9\times 10^{-3}$dexarcmin$^{-1}$ or $\sim-0.29$dexkpc$^{-1}$), while almost no gradient in the outskirts, hinting for an ex-situ halo or for an ``outside-in’’ star formation. Although DESI [$α$/Fe] ratios for Sextans stars with [Fe/H]$\gtrsim-2.0$ are in line with literature values, those for very metal-poor stars ([Fe/H]$\lesssim-2.0$) present a large scatter and strong anti-correlation with metallicity, warranting a caution for using DESI abundances in this regime. We find one ultra metal-poor candidate in CanesVenatici~I, which requires higher resolution spectroscopic follow-ups.

💡 Research Summary

This paper presents a comprehensive analysis using the first data release (DR1) from the Dark Energy Spectroscopic Instrument (DESI) to confirm stellar membership in dwarf galaxies within the Local Group. The study cross-matches DESI spectroscopic targets with pre-existing candidate member lists based on Gaia astrometry, photometry, and proper motions. The key addition from DESI is precise radial velocities, which, when combined with the Gaia data, allow for secure membership determination.

The authors apply a three-tiered selection process to identify member stars. First, they apply basic cuts using Gaia parallax and DESI surface gravity to remove obvious foreground Milky Way stars. Second, they require stars to align with the expected color-magnitude diagram isochrone of the galaxy, and to have proper motions and radial velocities consistent with the systemic motion of the galaxy within a defined threshold. Finally, they cross-match the surviving stars with Bayesian probability-based candidate member catalogs from Battaglia et al. (2022) and Pace et al. (2022), confirming stars with high membership probability.

This methodology successfully confirms member stars in 16 different systems observed by DESI DR1, spanning ultra-faint dwarfs (e.g., Bootes II, Hercules), classical dwarfs (e.g., Draco, Sextans), and irregular galaxies (e.g., IC 1613). The largest numbers of confirmed members are found for Draco and Sextans. The study notes that while Draco has been analyzed in detail in other works using DESI data, they focus their in-depth analysis on the Sextans dwarf spheroidal galaxy.

For Sextans, the research leverages DESI data to trace stellar kinematics and chemical properties out to unprecedentedly large radii, up to approximately 10 half-light radii. The derived systemic velocity and velocity dispersion are consistent with previous studies. A major finding is the clear dichotomy in the stellar populations based on metallicity. The more metal-poor population (