The Keck/DEIMOS Stellar Archive: I. Uniform Velocities and Metallicities for 78 Milky Way Dwarf Galaxies and Globular Clusters

We present a homogeneous spectroscopic dataset of 22,339 stars in 78 Milky Way dwarf galaxy satellites and globular clusters. All data were taken with the Keck II telescope and Deep Extragalactic Imaging Multiobject Spectrograph (DEIMOS) spectrograph using the 1200G grating (spectral resolution R~6000). Based on a uniform data reduction of 411 DEIMOS masks, we present a catalog of individual stellar radial velocities, equivalent width-based [Fe/H] metallicities, and membership estimates. The Milky Way satellites range from M_V = 2 to -14 (M* = 10^1.5 to 10^7.5 Msun); the majority of individual stars presented in these systems have magnitudes 17 > r > 22. The data were reduced to 1D spectra using PypeIt, which provides near Poisson statistics-level sky subtraction. Radial velocities were determined via dmost, a forward modeling method first presented here, which combines both synthetic telluric and stellar templates to determine stellar radial velocities. We assess the accuracy and precision our method via comparison to thousands of repeat measurements and literature values. We determine a velocity error floor of 1.1 km/s and a CaII triplet-based metallicity error floor of 0.1 dex. We calculate internal velocity dispersions and compare to literature values, demonstrating 20-50% improved precision over the literature in most cases. In a companion paper, we use our homogeneous catalogs to explore properties of these Milky Way satellites, including previously unpublished measurements in several systems including Bootes II and Draco II. We provide full access to the data catalogs to enable further studies.

💡 Research Summary

This paper presents a homogeneous spectroscopic dataset of 22,339 stars observed in 78 Milky Way satellite systems (35 dwarf galaxies and 43 globular clusters or unclassified objects) using the Keck II telescope’s DEIMOS spectrograph with the 1200 g/mm grating (R ≈ 6000). The authors retrieved 411 multi‑object slitmask exposures from the Keck Observatory Archive spanning 2003–2023, applied a uniform reduction pipeline with the open‑source code PypeIt (v1.10), and derived consistent radial velocities, Ca II‑triplet based metallicities, and membership probabilities.

Key methodological advances include: (1) a rigorous 2‑D reduction that combines the eight DEIMOS CCDs into four mosaics, uses a sixth‑order Legendre polynomial for wavelength calibration (median RMS = 0.06 pixel ≈ 0.02 Å, corresponding to ≈0.75 km s⁻¹), and performs two‑stage sky subtraction with optimal extraction; (2) the introduction of “dmost”, a forward‑modeling velocity estimator that simultaneously fits synthetic stellar and telluric templates, thereby minimizing systematic shifts from atmospheric absorption and sky lines. Validation against thousands of repeat observations and literature values yields a velocity error floor of 1.1 km s⁻¹. (3) Equivalent‑width based