The dark matter content of Milky Way dwarf spheroidal galaxies: Draco, Sextans and Ursa Minor

TL;DR

This study uses axisymmetric Jeans Anisotropic Multi-Gaussian Expansion (JAM) modeling to infer the inner dark matter distributions of three Milky Way dwarf spheroidal galaxies (Draco, Sextans, Ursa Minor) from DESI Milky Way Survey stellar kinematics, employing both a traditional single-population approach and a chemodynamical two-population framework. By combining DESI MWS data with Walker2023 observations and decomposing tracer densities via Multi-Gaussian Expansions, the authors constrain the inner density slope $\gamma$ and compute astrophysical $J$ and $D$ factors, finding a diversity of inner profiles across the galaxies (e.g., $\gamma$ ranging roughly from ~0.3 to ~0.7) with results broadly consistent between modeling schemes. The chemodynamical model reveals distinct metal-rich and metal-poor populations that are more centrally concentrated and dynamically colder than one another, though some velocity-dispersion fits hint at departures from steady-state, likely due to non-equilibrium effects. Overall, the work demonstrates the sensitivity of inner DM inferences to modeling choices and data selection, and provides updated $J$ and $D$ factors while highlighting ongoing uncertainties in dSph dark matter content and the core-cusp problem.

Abstract

The Milky Way Survey of the Dark Energy Spectroscopic Instrument (DESI) has so far observed three classical dwarf spheroidal galaxies (dSphs): Draco, Sextans and Ursa Minor. Based on the observed line-of-sight velocities and metallicities of their member stars, we apply the axisymmetric Jeans Anisotropic Multi-Gaussian Expansion modeling (JAM) approach to recover their inner dark matter distributions. In particular, both the traditional single-population Jeans model and the multiple population chemodynamical model are adopted. With the chemodynamical model, we divide member stars of each dSph into metal-rich and metal-poor populations. The metal-rich populations are more centrally concentrated and dynamically colder, featuring lower velocity dispersion profiles than the metal-poor populations. We find a diversity of the inner density slopes $γ$ of dark matter halos, with the best constraints by single-population or chemodynamical models consistent with each other. The inner density slopes are $0.71^{+0.34}_{-0.35}$, $0.26^{+0.22}_{-0.12}$ and $0.33^{+0.20}_{-0.16}$ for Draco, Sextans and Ursa Minor, respectively. We also present the measured astrophysical J and D factors of the three dSphs. Our results indicate that the study of the dark matter content of dSphs through stellar kinematics is still subject to uncertainties behind both the methodology and the observed data, through comparisons with previous measurements and data sets.

Figures (13)

• Figure 1: Top: A comparison between the Draco member star LOSV measurements by DESI MWS ($y$-axis) and Walker2023 ($x$-axis). The red line is the best fit with slope of 1, and its $y$-intercept is the offset between two observations (see the legend). Bottom: The histogram of the differences between the Draco member star LOSV measurements by DESI MWS and Walker2023. The red dashed vertical line represents the offset between two observations determined from the fit in the top panel.
• Figure 2: Distributions of LOSV errors for Draco member stars, observed by DESI MWS (blue) or by Walker2023 (red). The black vertical dashed line is the intrinsic LOSV dispersion of Draco (see Table \ref{['tab:dwarf prop']}).
• Figure 3: Top panel: Surface number density profiles of targets and spectroscopically observed member stars along the major axis, $x'$, for each dSph. The black squares and triangles are mean surface number densities in elliptical isophotal radial bins of targets and spectroscopically observed member stars. The colored dashed curves represent different MGE components, and the black solid curves are the summations of these MGE components, which match well the black squares. Bottom panel: The completeness fraction of spectroscopically observed member stars versus targets, as a function of the projected distance along the major axis. The left, middle and right panels are for Draco, Sextans and UMi, respectively.
• Figure 4: Surface number density profiles of two template maps along the major axis, $x'$, for each dSph. The two template maps are obtained for the metal-rich (red) and metal-poor (blue) subpopulations after a hard cut in metallicity for the division, and the hard cut is chosen to maximize the difference in the half-number radii of the two populations. Red and blue dots are surface number densities directly calculated from corresponding template maps. Solid curves with the same colors as dots denote the reconstructed profiles from different MGEs. The left, middle and right panels are for Draco, Sextans and UMi, respectively.
• Figure 5: Best-fit model parameters for the single-population (magenta) and chemodynamical (green) models, with the three rows refer to Draco, Sextans and UMi. The left and right green arrows in the first two columns for parameters $\lambda$ and $\kappa$ represent results for the metal-poor and metal-rich populations separated by the chemodynamical model. The $y$-axis ranges for different parameters are not the same. The meanings of different model parameters can be found in Section \ref{['sec:method']}.