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

TL;DR

This study utilizes DESI DR1 data to identify and analyze stellar members of Local Group dwarf galaxies, confirming Gaia-based selection effectiveness and providing new insights into Sextans' stellar kinematics, metallicity gradients, and ultra metal-poor candidates.

Contribution

First application of DESI DR1 data to confirm membership and analyze stellar populations in Local Group dwarf galaxies, including detailed kinematic and metallicity studies of Sextans.

Findings

Confirmed membership in 16 dwarf galaxies using DESI radial velocities.

Extended stellar kinematic analysis of Sextans to large radii, consistent with previous results.

Identified a potential ultra metal-poor star candidate in Canes Venatici I.

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 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$10~half-light radii). Our derived systemic velocity and velocity dispersion are consistent with previous works. We find that the more metal-poor population exhibits a higher velocity dispersion and extends to larger radii, whereas the more metal-rich population is kinematically colder and centrally concentrated. The metallicity gradient is steeper in the inner regions of Sextans ($\sim -9\times 10^{-3}$~dex~arcmin$^{-1}$ or $\sim-0.29$~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 [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 Canes~Venatici~I, which requires higher resolution spectroscopic follow-ups.