[Paper Review] On the formation and evolution of the globular cluster Omega Centauri

G. Carraro, Cesario Lia|arXiv (Cornell University)|Mar 24, 2000

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper proposes that Omega Centauri formed as an isolated, high-redshift dwarf elliptical galaxy that evolved through self-enrichment and was later captured by the Milky Way, using N-body/hydrodynamical simulations to reproduce its observed structure, kinematics, and metallicity spread. The model successfully reproduces the cluster's core radius, velocity dispersion, and bimodal metallicity distribution, and predicts an extended dark matter halo influencing stars beyond 20 arcminutes.

ABSTRACT

By means of N-body/hydrodynamical simulations we model the evolution of a primordial 10^{8} solar masses density peak which ends up in an object closely resembling the present day globular cluster Omega Centauri. We succeed to reproduce the main features of the cluster, namely the structure, kinematics and metallicity distribution. We suggest that Omega Centauri might be a cosmological dwarf elliptical, formed at high redshift, evolved in isolation and self-enriched, and eventually fallen inside the potential well of the Milky Way, in agreement with the Searle-Zinn (1978) paradigm for galactic globular clusters formation. We finally suggest that Omega Centauri is probably surrounded by an extended Dark Matter (DM) halo, for which no observational evidence is at present available. We expect that signatures, if any, of the DM halo can be found in the kinematics of stars outside about 20 arcmin.

Motivation & Objective

• To investigate whether Omega Centauri could be the remnant of a dwarf elliptical galaxy rather than a typical globular cluster.
• To model the formation and evolution of Omega Centauri using N-body and smoothed particle hydrodynamics (SPH) techniques.
• To reproduce the cluster's observed structural, kinematical, and chemical properties, including its metallicity spread.
• To test the hypothesis that Omega Centauri is surrounded by an extended dark matter halo influencing its outer stellar kinematics.
• To assess the consistency of the model with the Searle-Zinn paradigm of globular cluster formation from dwarf galaxies.

Proposed method

• Simulations were performed using a Tree-SPH code with adaptive time-steps and variable smoothing lengths for each particle.
• The initial setup included a $9 \times 10^7 M_\odot$ dark matter halo with a $10^{-4}$ metallicity gas component, distributed homogeneously within the potential well.
• Star formation was modeled using a metallicity-dependent cooling efficiency and feedback from Type II and Ia supernovae and massive star winds.
• Chemical enrichment was tracked via a closed-box model applied to individual gas particles, allowing for self-enrichment over time.
• The gravitational forces were computed using a hierarchical tree algorithm with $\theta = 0.8$ and quadrupole expansion.
• The simulation evolved until virial equilibrium was reached, then followed the formation of stars and the development of the cluster's final structure.

Experimental results

Research questions

• RQ1 Can a self-enriching, isolated dwarf elliptical galaxy reproduce the observed structural and kinematical properties of Omega Centauri?
• RQ2 Does the metallicity distribution function (MDF) of Omega Centauri arise naturally from a prolonged, irregular star formation history in a self-enriching system?
• RQ3 Is the observed high central velocity dispersion and low rotational speed in Omega Centauri consistent with a model of isolated formation and evolution?
• RQ4 What is the expected dark matter distribution around Omega Centauri, and can it be probed through stellar kinematics beyond 20 arcminutes?
• RQ5 Does the tidal radius of Omega Centauri in the model match observations, and if not, what physical processes might explain the discrepancy?

Key findings

• The simulated cluster reproduces the observed core radius of 2.58 arcmin and concentration of 1.24, matching the structural parameters of Omega Centauri.
• The central velocity dispersion in the model is 13 km/s, slightly lower than the observed 17 km/s, indicating a somewhat colder system.
• The model produces a bimodal metallicity distribution with a main peak at [Fe/H] ≈ -1.75 and a secondary peak at [Fe/H] ≈ -1.2, four times smaller, consistent with observations.
• A significant number of stars with [Fe/H] ≈ -0.5 are formed, indicating late-stage, high-metallicity star formation.
• The tidal radius in the model is 65 arcmin, significantly larger than the observed value, likely due to missing tidal stripping during Milky Way accretion.
• An extended dark matter halo is predicted, with dark matter dominating beyond 20 arcmin (transition radius), potentially affecting stellar kinematics in the outer regions.