Intra-cluster light as a dynamical clock for galaxy clusters: Insights from the MAGNETICUM, IllustrisTNG, Hydrangea, and Horizon-AGN simulations

¹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
² School of Physics, University of New South Wales, NSW 2052, Australia
³ Max Planck Institute for Astrophysics, D-85748 Garching, Germany
⁴ Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
⁵ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
⁸ Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
⁹ Department of Physics and Astronomy, University of Waterloo, Waterloo Ontario N2L3G1, Canada,
¹⁰ OCA, P.H.C Boulevard de l’Observatoire CS 34229, 06304 Nice Cedex 4, France
¹¹ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
¹² Instituto de Astrofísica de Andalucía–CSIC, Glorieta de la Astronomía s/n, E–18008 Granada, Spain
¹³ Observatório Nacional, Rua General José Cristino, 77 – Bairro Imperial de São Cristóvão, Rio de Janeiro 20921-400, Brazil
¹⁴ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁵ Max Planck Institute for Extraterrestrial Physics, Giessenbachstr. 1, 85748 Garching, Germany
¹⁶ Instituto de Astrofísica de Canarias, Vía Láctea S/N, E-38205 La Laguna, Spain
¹⁷ Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Spain
¹⁸ , 10900 Euclid Avenue Cleveland OH 44106, USA

^⋆ Corresponding author: lkimmig@usm.lmu.de

Received: 26 March 2025
Accepted: 2 June 2025

Abstract

Context. As the most massive nodes of the cosmic web, galaxy clusters represent the best probes of structure formation. Over time, they grow by accreting and disrupting satellite galaxies, adding those stars to the brightest cluster galaxy (BCG) and the intra-cluster light (ICL). However, the formation pathways of galaxy clusters can vary significantly.

Aims. To inform upcoming large surveys, we aim to identify observables that can distinguish galaxy cluster formation pathways.

Methods. Using four different hydrodynamical simulations, Magneticum, TNG100 of IllustrisTNG, Horizon-AGN, and Hydrangea, we studied how the fraction of stellar mass in the BCG and ICL (f_{ICL + BCG}) relates to the galaxy cluster mass assembly history.

Results. For all simulations, f_{ICL + BCG} is the best tracer for the time at which the cluster has accumulated 50% of its mass (z_form), performing better than other typical dynamical tracers, such as the subhalo mass fraction, the halo mass, and the position offset of the cluster mass barycenter to the BCG. More relaxed clusters have a higher f_{ICL + BCG}, in rare cases up to 90% of all stellar mass, while dynamically active clusters have lower fractions, down to 20%, which we find to be independent of the exact implemented baryonic physics. We determine the average increase in f_{ICL + BCG} from stripping and mergers to be between 3–4% per gigayear. Furthermore, f_{ICL + BCG} is tightly traced by the stellar mass ratio between the BCG and both the second (M12) and fourth (M14) most massive cluster galaxy. The average galaxy cluster has assembled half of its halo mass by z_form = 0.67 (about 6 gigayears ago), though individual histories vary significantly from z_form = 0.06 to z_form = 1.77 (0.8–10 gigayears ago).

Conclusions. As all four cosmological simulations consistently find that f_{ICL + BCG} is an excellent tracer of the cluster dynamical state, upcoming surveys can leverage measurements of f_{ICL + BCG} to statistically quantify the assembly of the most massive structures through cosmic time.