The Keck/DEIMOS Stellar Archive: II. Dynamical Masses and Metallicities for a Uniform Sample of Milky Way Satellites

TL;DR

This study delivers the largest self-consistent, homogeneous set of spectroscopic measurements for Milky Way satellites, drawing from Keck/DEIMOS data re-analyzed uniformly. By deriving internal velocity dispersions, enclosed dynamical masses, mean metallicities, and metallicity dispersions for 67 systems (with 10+ members), it demonstrates clear distinctions between satellite galaxies and globular clusters in dynamical mass and mass-to-light ratios. The work reveals a break in the stellar mass–metallicity relation around $\log M_*/M_\odot \approx 4$, and finds satellite metallicity dispersions of $\sim$0.3–0.4 dex, with a CDM-consistent trend of rising $M_{1/2}/L_{1/2}$ at lower stellar mass and increasing densities for fainter satellites. These uniform measurements sharpen constraints on the stellar mass–halo mass relation, improve J-factor estimates for dark matter searches, and lay groundwork for interpreting the forthcoming deluge of Milky Way satellites from LSST, Roman, and Euclid.

Abstract

Population-level studies of Milky Way satellites used to constrain dark matter or the threshold of galaxy formation often rely on velocity dispersions and metallicities derived from heterogeneous spectroscopic analyses. Systematic differences between data reduction pipelines and membership criteria can masquerade as astrophysical signals, or obscure real trends. Here, we present the largest self-consistent sample of spectroscopically-derived quantities for Milky Way satellite galaxies and globular clusters based on a homogeneous re-analysis of individual stars observed with the Keck/DEIMOS spectrograph. We determine enclosed dynamical masses, mean [Fe/H] metallicities, and metallicity dispersions for 67 systems with 10 or more member stars. At a given stellar mass, systems classified as satellite galaxies are well separated from globular clusters in their dynamical mass and mass-to-light ratios. The average enclosed mass densities of satellite galaxies agree with semi-analytic CDM model predictions. For satellite galaxies, we observe a break in the stellar mass-metallicity relation near log M*/Msun = 4 (M_V ~ -4.5). Above this stellar mass, satellite galaxies show the well-known tight trend (0.16 dex scatter in [Fe/H]) of decreasing metallicity with stellar mass; below log M*/Msun = 4, the mass-metallicity relation flattens and/or increases in scatter. Satellite galaxies have internal metallicity scatter between 0.3-0.4 dex across our stellar mass range. These uniform measurements will enable tighter constraints on the stellar mass-halo mass relation, improved J-factor estimates for dark matter searches, and lay a foundation for interpreting the flood of new Milky Way satellites expected in the LSST/Roman/Euclid era.

Figures (9)

• Figure 1: Left: Absolute magnitude ($M_V$) versus half-light radius ($r_{\rm 1/2}$) for confirmed Milky Way satellite galaxies (red circles), globular/star clusters (blue diamonds) and ambiguously classified objects (purple squares), as compiled in v1.0.6 of pace2024. Solid symbols are systems with uniform DEIMOS measurements included in this work; open symbols indicate Milky Way systems without DEIMOS coverage. Thin diagonal lines of constant effective surface brightness are shown from top to bottom: 26, 28, 30 mag arcsec$^{-2}$. Right: Velocity dispersion ($\sigma_v$) versus $M_V$ as measured by this work (solid symbols), and system with non-DEIMOS literature measurements (open symbols). While the DEIMOS sample represents a fraction of all known MW systems ( left), it includes the majority of systems with measured internal velocity dispersions ( right).
• Figure 2: Left: Dynamical mass enclosed within the half-light radius ($M_{\rm 1/2}$) versus stellar mass ($\log M_\star$). Right: Mass-to-light ratio (M$_{\rm 1/2} / L_{1/2}$) versus stellar mass. In both panels, the top x-axis shows absolute magnitude ($M_V$). At a given stellar mass, satellite galaxies (red circles) have larger dynamical mass and higher mass-to-light ratios compared to globular clusters (blue diamonds). Globular clusters are consistent with baryonic-only mass. Satellite galaxies show decreasing enclosed dynamical mass and increasing M$_{\rm 1/2} / L_{1/2}$ with decreasing stellar mass.
• Figure 3: Left: The average enclosed mass density ($\overline{\rho}_{1/2}$) plotted against orbital pericenter (R$_{\rm peri}$). The black line represents twice the enclosed Milky Way density as a function of radius (2$\overline{\rho}_{\rm MW}$). Systems near this line are expected to show signs of tidal disruption. While most systems are far from this threshold, we label systems close to the disruption line. Right: We plot $\overline{\rho}_{1/2}$ against total stellar mass. For satellite galaxies, lower luminosity systems tend to have higher enclosed mass density.
• Figure 4: Left: The mean stellar metallicity ($\overline{\rm [Fe/H]}$) and ( right) the internal metallicity spread ($\sigma_{\rm [Fe/H]}$) versus total stellar mass. Systems are color-coded as previous figures. Satellite galaxies show the well-known mass-metallicity relationship down to $\log\;[M_\star/{\rm M}_\odot] \approx 4$. Below this stellar masses threshold, the mass-metallicity relationship for satellite galaxies flattens and/or increases scatter. The internal metallicity scatter shows a similar upturn or increased scatter at this stellar mass. The red star symbol at $\log\;[M_\star/{\rm M}_\odot] \approx 7.6$ is the disrupting Sagittarius dSph which we do not included in our fits. In contrast, globular clusters show no mass-metallicity relation and have internal metallicity dispersions below 0.2 dex.
• Figure 5: Combining kinematic and metallicity measurements, we plot the mass-to-light ratio (M$_{\rm 1/2} / L_{1/2}$) versus the half-light radius ($r_{\rm 1/2}$). Symbols are color-coded based on their internal metallicity dispersion ($\sigma_{\rm [Fe/H]}$). Systems with resolved metallicity dispersions are shown with black outlines. Satellite galaxies (circles) and star clusters (diamonds) are well separated in these properties.