[Paper Review] N-Body Simulations of Small Galaxy Groups

H. Aceves, Héctor Velázquez|arXiv (Cornell University)|Aug 17, 2002

Astronomy and Astrophysical Research3 citations

TL;DR

This study uses N-body simulations to investigate the dynamics of small galaxy groups, showing that compact groups like Hickson's can form naturally from diffuse initial conditions without requiring a primordial common dark halo. The key finding is that ~40% of virialized groups survive without complete merging for ~10 Gyr, resolving the overmerging problem and suggesting compact groups and clusters have similar mass-to-light ratios.

ABSTRACT

A series of N-body simulations aimed to study the dynamics of small groups of galaxies are presented. The properties of small galaxy groups are very well reproduced, and those of Hickson's compact groups are well reproduced by the most advanced stage of collapsing groups. We find no overmerging problem in our simulations. An important fraction of groups (~40%) initially in virial equilibrium can last for ~10 Gyr without complete merging. These results provide an alternative solution to the overmerging expected in Hicksons's compact groups.

Motivation & Objective

• To investigate the dynamical evolution of small galaxy groups using N-body simulations.
• To test whether Hickson's compact groups (HCG) can form and survive without requiring a primordial common dark halo.
• To resolve the overmerging problem—why HCG appear long-lived despite short crossing times.
• To compare simulated group properties with observed median values from catalogs like CfA, GCF, and MK.
• To assess the role of initial conditions (maximum expansion vs. virial equilibrium) in group evolution and longevity.

Proposed method

• Conduct N-body simulations of small galaxy groups with 4–6 galaxies, using gravitational forces and particle-based dynamics.
• Implement two initial conditions: 'maximum expansion' (diffuse, non-virialized) and virial equilibrium (previously relaxed).
• Use no primordial common dark halo; instead, simulate dark matter halos around individual galaxies.
• Evolve systems over ~10 Gyr to assess survival, merging, and dynamical properties.
• Apply mass estimators (median mass estimator and virial mass estimator) to compare mass estimates.
• Compare simulated group properties (velocity dispersion, radius, crossing time) with observational data from Hickson, CfA, GCF, and MK catalogs.

Experimental results

Research questions

• RQ1 Can compact galaxy groups form and remain stable without a primordial common dark halo?
• RQ2 Do initial conditions such as maximum expansion or virial equilibrium lead to realistic small group properties?
• RQ3 What fraction of initially virialized groups survive without complete merging over 10 Gyr?
• RQ4 How do the mass-to-light ratios of Hickson compact groups compare to those of galaxy clusters?
• RQ5 Is the overmerging problem in compact groups resolved by natural dynamical evolution from diffuse initial states?

Key findings

• Groups starting from maximum expansion reproduce the median dynamical properties of small galaxy groups from observed catalogs.
• Approximately 10% of initially diffuse groups evolve into configurations matching the median properties of Hickson compact groups.
• About 40% of groups initially in virial equilibrium survive without complete merging for ~10 Gyr, indicating long-term stability.
• The median mass estimator (MME) provides a more accurate mass estimate than the virial mass estimator (VME) for present-day groups.
• The mass-to-light ratio of Hickson compact groups is estimated at ~250 solar units, suggesting similar baryonic-to-total mass fractions as galaxy clusters.
• No significant overmerging problem is observed; compact groups can remain stable due to natural dynamical evolution, not requiring fine-tuned initial conditions or a common dark halo.