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 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 [$α$/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 Canes~Venatici~I, which requires higher resolution spectroscopic follow-ups.

Figures (8)

• Figure 1: Properties of the Local Group galaxies seen in DESI DR1. Fist column: projected on sky position. Two black ellipses represent 5 and 10 half-light radii. Black markers denote the position of the system's centre. Second column: Gaia's colour magnitude diagram. An old (10 Gyr) very metal-poor (${\rm[Fe/H]}=-2.0$) PARSEC isochrone Bressan12 is represented with a black dashed line. Third column: Gaia proper motion space. Green circle marks the systemic proper motion. Forth column: metallicity vs radial velocity space. The three vertical dashed-lines denote the systemic line-of-sight velocity $\pm$5 times its velocity dispersion.
• Figure 2: (continued) Properties of the Local Group galaxies seen in DESI DR1.
• Figure 3: (continued) Properties of the Local Group galaxies seen in DESI DR1.
• Figure 4: (continued) Properties of the Local Group galaxies seen in DESI DR1.
• Figure 5: Radial velocities and metallicities in Sextans. Panels A: RVs as a function of the projected elliptical distance. Panels B: [Fe/H] as a function of the projected elliptical distance. Panels C: [Fe/H] vs RVs. Panels D: RVs' histogram. Panels E: metallicities' histogram. Upper five panels are for the sample$-1$, i.e. Sextans members as described in Section \ref{['sec:crit']}. Lower panels are for the sample$-2$, similar but excluding stars with large uncertainties on the metallicities, $\sigma_{{\rm[Fe/H]}} > 1.0$ dex. In both samples, stars with $\sigma_{{\rm[Fe/H]}} > 0.5$ dex are shown with errorbars. Systemic radial velocity and velocity dispersion of Sextans are derived and reported for the sample$-2$.