Constraints on MACHO Dark Matter from Compact Stellar Systems in Ultra-Faint Dwarf Galaxies (1605.03665v2)

Abstract: I show that a recently discovered star cluster near the center of the ultra-faint dwarf galaxy Eridanus II provides strong constraints on massive compact halo objects (MACHOs) of >~5 M_sun as the main component of dark matter. MACHO dark matter will dynamically heat the cluster, driving it to larger sizes and higher velocity dispersions until it dissolves into its host galaxy. The stars in compact ultra-faint dwarf galaxies themselves will be subject to the same dynamical heating; the survival of at least ten such galaxies places independent limits on MACHO dark matter of masses >~10 M_sun. Both Eri II's cluster and the compact ultra-faint dwarfs are characterized by stellar masses of just a few thousand M_sun and half-light radii of 13 pc (for the cluster) and ~30 pc (for the ultra-faint dwarfs). These systems close the ~20--100 M_sun window of allowed MACHO dark matter and combine with existing constraints from microlensing, wide binaries, and disk kinematics to rule out dark matter composed entirely of MACHOs from ~10$^{-7}$ M_sun up to arbitrarily high masses.

• The paper constrains MACHO dark matter by modeling dynamical heating in Eridanus II’s star cluster using the Fokker–Planck approach.
• It finds that MACHO masses between 20–100 M⊙ would have disrupted the observed cluster structure, suggesting these objects are not a major dark matter component.
• The study underscores the value of ultra-faint dwarf galaxies in refining dark matter models and motivates future precision observations.

Constraints on MACHO Dark Matter from Ultra-Faint Dwarf Galaxies

The paper discusses the constraints on Macho dark matter posited by the existence and characteristics of compact stellar systems within ultra-faint dwarf galaxies, with a particular focus on the ultra-faint dwarf galaxy Eridanus II. Massive Compact Halo Objects (MACHOs) are theorized as potential constituents of dark matter, particularly at mass scales that are not easily probed by other forms of detection like microlensing. This paper leverages the dynamical properties of star clusters in dwarf galaxies to place stringent limits on the presence of MACHO dark matter.

Key Findings and Methodology

The paper examines a star cluster near the center of Eridanus II to place constraints on MACHO masses of 5 $M_\odot$ or greater. The research narrative is built upon the dynamical heating hypothesis: if dark matter consists largely of MACHOs, these massive bodies would impart energy to stellar systems, causing them to expand over time until dissolution. Employing the Fokker-Planck approximation, the diffusion of kinetic energy in stellar systems affected by MACHOs was modeled, particularly focusing on Eridanus II's cluster. Parameters describing cluster expansion provide a timescale for the process, with MACHO dark matter heating potentially leading to observable changes in the cluster's structure.

The paper further examines other ultra-faint dwarf galaxies to support its conclusions, considering the broader population characterized by relatively low stellar masses and smaller half-light radii. It utilizes constraint calculations based on their observed characteristics in conjunction with theoretical dynamical models to corroborate findings from Eridanus II.

Strong Numerical Results

• The paper constrains MACHO masses between approximately 20 to 100 $M_\odot$, effectively ruling out MACHOs in this mass range as a significant component of dark matter. Beyond this range, the combination of microlensing, wide binary star dynamics, and kinematic studies of the Galactic disk effectively extend constraints up to higher masses.
• Eridanus II’s star cluster, characterized by an approximate mass of 2000-6000 $M_\odot$ (depending on its age, between 3 and 12 Gyr), and a half-light radius of 13 pc, is determined to provide substantial limits on MACHO presence. If MACHOs comprised a significant fraction of dark matter, the cluster would be expected to exhibit larger sizes and potentially dissolve over its lifetime of several billion years.

Implications and Future Directions

This paper solidifies the utility of ultra-faint dwarf galaxies in constraining the nature of dark matter, especially in regions of parameter space where traditional methods like microlensing falter. Results strongly suggest that MACHOs form a negligible portion of dark matter, pressuring alternative dark matter hypotheses to account for ultra-faint galaxy dynamics without resorting to large populations of massive MACHOs.

The implications further extend to the refinement of dark matter density profiles and velocity dispersions within dwarf galaxies, enriching our understanding of dark matter distribution at galactic scales. Future studies may expand upon these results by employing high-precision photometric and spectroscopic observations to refine age, mass, and structural parameters of clusters in such galaxies, potentially strengthening the constraints further or exploring new parameter spaces.