Populating a cluster of galaxies - I. Results at \fontshape{it}{z}=0

TLDR

We simulate the formation of a massive rich cluster in a flat, low‑density universe, following the collapse, star‑formation history, and orbital dynamics of all galaxies brighter than the Fornax dwarf while consistently tracking dark‑halo structure for galaxies brighter than the SMC, using ~2×10⁷ dark‑matter particles and ~5,000 resolved galaxies, and validating numerical convergence across multiple resolutions with a baryonic model tuned to match isolated spiral properties. The simulations reproduce observed cluster properties—including the luminosity function, mass‑to‑light ratios, luminosity, number and velocity‑dispersion profiles, and morphology–radius relation—and show that explicit galaxy merging quantitatively explains the cluster’s bulge and elliptical populations.

Abstract

We simulate the assembly of a massive rich cluster and the formation of its constituent galaxies in a flat, low-density universe. Our most accurate model follows the collapse, the star formation history and the orbital motion of all galaxies more luminous than the Fornax dwarf spheroidal, while dark halo structure is tracked consistently throughout the cluster for all galaxies more luminous than the SMC. Within its virial radius this model contains about 2 × 107 dark matter particles and almost 5000 distinct dynamically resolved galaxies. Simulations of this same cluster at a variety of resolutions allow us to check explicitly for numerical convergence both of the dark matter structures produced by our new parallel N-body and substructure identification codes, and of the galaxy populations produced by the phenomenological models we use to follow cooling, star formation, feedback and stellar aging. This baryonic modelling is tuned so that our simulations reproduce the observed properties of isolated spirals outside clusters. Without further parameter adjustment our simulations then produce a luminosity function, a mass-to-light ratio, luminosity, number and velocity dispersion profiles, and a morphology–radius relation which are similar to those observed in real clusters. In particular, since our simulations follow galaxy merging explicitly, we can demonstrate that it accounts quantitatively for the observed cluster population of bulges and elliptical galaxies.