Galactic Dynamics and Local Dark Matter

The concordance Lambda Cold Dark Matter (Lambda-CDM) model for the formation of structure in the Universe, while remarkably successful at describing observations of structure on large scales, continues to be challenged by observations on galactic scales. Fortunately, CDM models and their various proposed alternatives make a rich variety of testable predictions that make the Local Group and our own Milky Way Galaxy key laboratories for exploring dark matter (DM) in this regime. However, some of the most definitive tests of local DM require microarcsecond astrometry of faint sources, an astrometric regime that is a unique niche of SIM Lite. This chapter explores the important and distinct contributions that can be made by SIM Lite in the exploration of galaxy dynamics and DM on galaxy scales and that have cosmological consequences. Key areas of potential SIM Lite exploration include (1) measuring the shape, orientation, density law, and lumpiness of the dark halo of the Milky Way and other nearby galaxies, (2) determining the orbits of Galactic satellites, which may be representatives of late infall from the hierarchical formation of the Milky Way, (3) ascertaining the distribution of angular momentum and orbital anisotropy of stars and globular clusters to the outer reaches of the Galactic halo, dynamical properties that hold clues to the early hierarchical formation of the Galaxy, (4) measuring the physical nature of DM by placing strong constraints on the phase space density in the cores of nearby dSph galaxies, and (5) reconstructing the dynamical history of the Local Group through the determination of orbits and masses of its constituent galaxies.

💡 Research Summary

The chapter presents a compelling case for the unique scientific contributions that the Space Interferometry Mission Lite (SIM Lite) can make to the study of dark matter (DM) on galactic and Local‑Group scales. While the ΛCDM (Lambda Cold Dark Matter) paradigm successfully explains the large‑scale structure of the Universe, it faces persistent challenges on the scale of individual galaxies—most notably the core‑cusp discrepancy, the “missing satellites” problem, and tensions in the outer‑halo kinematics of the Milky Way. Resolving these issues requires direct, high‑precision measurements of the three‑dimensional motions of faint stars, globular clusters, dwarf spheroidal (dSph) galaxies, and satellite systems, a regime where only micro‑arcsecond astrometry can provide decisive constraints.

SIM Lite’s capability to deliver proper motions with accuracies of 1–10 μas yr⁻¹ for objects as faint as V ≈ 20–22 opens a new observational window. The chapter outlines five inter‑related research thrusts that exploit this capability:

1. Mapping the Milky Way’s Dark Halo – By obtaining precise proper motions for thousands of distant RR Lyrae, blue horizontal‑branch stars, and the central stars of satellite galaxies, SIM Lite will directly measure the halo’s shape (oblateness vs. triaxiality), orientation, radial density profile, and sub‑halo lumpiness. This will move halo modeling beyond indirect inferences from line‑of‑sight velocities and gas dynamics, allowing a decisive test of whether the halo is spherical, flattened, or exhibits a preferred axis.

2. Reconstructing Satellite Orbits – ΛCDM predicts that many Milky Way satellites are the remnants of late‑infall subhalos. However, observed satellite planes and anisotropies suggest a more complex accretion history. SIM Lite will determine full three‑dimensional orbits for all classical and ultra‑faint dwarfs, quantifying eccentricities, inclination angles, and pericentric passages. These data will reveal whether satellites are truly recent infall objects, products of group infall, or have been dynamically reshaped by past major mergers.

3. Probing Angular Momentum and Anisotropy in the Outer Halo – The outer stellar halo retains the memory of hierarchical assembly. Precise proper motions for halo tracers beyond 50 kpc (e.g., red giants, BHB stars, and distant globular clusters) will enable reconstruction of the full six‑dimensional phase space, allowing measurement of the orbital anisotropy parameter β and the distribution of specific angular momentum. Comparisons with cosmological simulations will test predictions of radial versus tangential bias and the presence of coherent streams or shells.

4. Constraining the Phase‑Space Density of Dark Matter in dSph Galaxies – The internal dynamics of dwarf spheroidals are dominated by dark matter, yet mass estimates suffer from the mass‑anisotropy degeneracy. SIM Lite’s micro‑arcsecond proper motions for hundreds of member stars in nearby dSphs (e.g., Sculptor, Fornax, Draco) will break this degeneracy by providing transverse velocity dispersions. The resulting constraints on the central phase‑space density will discriminate between cold, warm, or self‑interacting dark matter models and will determine whether dSph cores are truly cored or cusped.

5. Reconstructing the Dynamical History of the Local Group – By measuring proper motions of M31, M33, and a suite of smaller group members with ∼10 μas yr⁻¹ precision, SIM Lite will enable a full orbital reconstruction of the Local Group over the past several gigayears. This will yield robust estimates of the total mass distribution, the timing of past close encounters (e.g., the predicted future Milky Way–M31 collision), and the role of past mergers in shaping the present‑day configuration.

Collectively, these five thrusts form a coherent program that leverages SIM Lite’s astrometric precision to test the core predictions of ΛCDM and its alternatives on the scales where the theory is most vulnerable. By directly measuring the shape of the Milky Way halo, the full three‑dimensional orbits of satellites, the angular momentum structure of the outer halo, the phase‑space density of dark matter in dwarf galaxies, and the orbital history of the Local Group, SIM Lite will provide the empirical foundation needed to either confirm the standard cosmological model or point toward new physics in the dark sector. The chapter thus argues that micro‑arcsecond astrometry is not merely a technical achievement but a decisive scientific tool for advancing our understanding of dark matter, galaxy formation, and cosmology.