[Paper Review] What is a galaxy? How Cold is Cold Dark Matter? Recent progress in Near Field Cosmology

G. Gilmore, D. B. Zucker+7 more|Lancaster EPrints (Lancaster University)|Apr 11, 2008

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper establishes a clear observational dichotomy between star clusters and galaxies based on half-light radius, showing that stable systems fall into two distinct classes: clusters (rₕ < 35 pc) and galaxies (rₕ > 120 pc), with no stable systems in between. It argues that dark matter halos in dSph galaxies are cored with low central densities (~0.1 M⊙ pc⁻³), contradicting cold dark matter model predictions of central cusps.

ABSTRACT

There has been a vast recent improvement in photometric and kinematic data for star clusters, Ultra Compact dwarfs, galactic nuclei, and local dSph galaxies, with Subaru contributing substantially to the photometric studies in particular. These data show that there is a bimodal distribution in half-light radii, with stable star clusters always being smaller than 35pc, while stable galaxies are always larger than 120pc. We extend the previously known observational relationships and interpret them in terms of a more fundamental pair of intrinsic properties of dark matter itself: dark matter forms cored mass distributions, with a core scale length of greater than about 100pc, and always has a maximum central mass density with a narrow range. The dark matter in dSph galaxies appears to be clustered such that there is a mean volume mass density within the stellar distribution which has the very low value of about 0.1$M_{\odot}$ pc$^{-3}$. None of the dSphs displays kinematics which require the presence of an inner cusp, while in two dSphs there is evidence that the density profile is shallow (cored) in the inner regions. The maximum central dark matter density derived is model dependent, but is likely to have a mean value (averaged over a volume of radius 10pc) of about 0.1$M_{\odot}$ pc$^{-3}$, which is 5GeV/c$^2$cm$^{-3}$). Galaxies are embedded in dark matter halos with these properties; smaller systems containing dark matter are not observed.

Motivation & Objective

• To clarify the observational distinction between star clusters and galaxies using photometric and kinematic data.
• To investigate the structural and dynamical properties of dark matter in dSph galaxies.
• To test whether dark matter halos in dwarf spheroidals exhibit cored or cusped density profiles as predicted by cold dark matter models.
• To determine the intrinsic scale lengths and mass densities of dark matter in local dSph systems.
• To assess whether ultra-compact dwarfs (UCDs) are massive star clusters or low-mass galaxies, based on their dynamical mass-to-light ratios.

Proposed method

• Analysis of half-light radii and absolute magnitudes from deep photometric surveys (e.g., Subaru, SDSS) for star clusters, UCDs, dSphs, and galactic nuclei.
• Application of isotropic Jeans' equation to derive inner mass distributions from line-of-sight velocity dispersion data in dSph galaxies.
• Comparison of observed mass profiles with theoretical predictions from cold dark matter simulations, particularly the r⁻¹ cusp profile.
• Use of distribution function modeling to assess the preference for cored versus cusped dark matter profiles in dSph systems.
• Evaluation of dynamical mass-to-light ratios in UCDs to test for the presence of dark matter.
• Assessment of tidal effects on systems in the size gap (35–120 pc) using galactocentric distance and kinematic data.

Experimental results

Research questions

• RQ1 Is there a fundamental size dichotomy between star clusters and galaxies in the local universe?
• RQ2 Do dark matter halos in dSph galaxies exhibit cored or cusped density profiles?
• RQ3 What is the typical central dark matter density in dSph galaxies, and how does it compare to CDM model predictions?
• RQ4 Are ultra-compact dwarfs (UCDs) massive star clusters or low-mass galaxies with dark matter halos?
• RQ5 Can tidal disruption explain the existence of systems in the 35–120 pc size gap?

Key findings

• A robust bimodal distribution in half-light radius exists: star clusters are always smaller than ~35 pc, while galaxies are always larger than ~120 pc.
• No stable, non-tidal system is observed in the size gap between 35 pc and 120 pc, with the exception of Coma Berenices, which shows signs of tidal disruption.
• All dSph galaxies studied exhibit cored dark matter profiles, with no evidence for the r⁻¹ cusped profiles predicted by standard cold dark matter simulations.
• The central dark matter density in dSphs is consistently low, with a mean value of approximately 0.1 M⊙ pc⁻³ (or 5 GeV/c² cm⁻³) within a 10 pc radius.
• Ultra-compact dwarfs (UCDs) have dynamical mass-to-light ratios consistent with stellar populations alone, indicating no significant dark matter content.
• The core scale length of dark matter in dSphs is greater than ~100 pc, suggesting a fundamental scale in dark matter structure that is larger than expected for cold dark matter particles.