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

Abstract

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 15 systems. Our results confirm that Gaia-based selection algorithms are effective in minimising foreground contamination. Two stars are found to be associated with DES~J0225$+$0304; if this is the case, then it leads to the first determination of the systemic radial velocity (RV$_{\rm{sys}}=-150.0\pm7.0$~km~s$^{-1}$). Draco and Sextans are the galaxies with the largest number of members. We focus on Sextans and, for the first time with DESI, trace its stellar kinematics to large radii (up to $\sim$10~half-light radii). We find that the metal-poor population exhibits a higher velocity dispersion and extends to larger radii, whereas the metal-rich population is kinematically colder and centrally concentrated. The metallicity gradient is steeper in the inner regions of Sextans ($\sim -12\times 10^{-3}$~dex~arcmin$^{-1}$ or $\sim -0.36$~dex~kpc$^{-1}$), while almost no gradient in the outskirts, hinting for an ex-situ halo or for an ``outside-in'' star formation. Although DESI [$\alpha$/Fe] ratios for Sextans stars with $\FeH\gtrsim-2.0$ are in line with literature values, those for very metal-poor stars ($\FeH\lesssim-2.0$) present a large scatter and strong anti-correlation with metallicity, warranting a caution for using DESI abundances in this regime. With a less strict selection, we identify 8 ultra metal-poor ([Fe/H]~$< -4$) candidates that require higher signal-to-noise ratio spectroscopic observations to determine their metallicities.