Environmental dependence of AGN activity and star formation in galaxy clusters from Magneticum simulations

¹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
² Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
e-mail: gregor.rihtarsic@fmf.uni-lj.si
³ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁴ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany

Received: 12 July 2023
Accepted: 28 December 2023

Abstract

Context. The environment inside and on the outskirts of galaxy clusters has a profound impact on the star formation rate and active galactic nucleus (AGN) activity in cluster galaxies. While the overall star formation and AGN suppression in the inner cluster regions has been thoroughly studied in the past, recent X-ray studies also indicate that conditions on the cluster outskirts may promote AGN activity.

Aims. We investigate how the environment and the properties of host galaxies impact the levels of AGN activity and star formation in galaxy clusters. We aim to identify significant trends in different galaxy populations and suggest possible explanations.

Methods. We studied galaxies with stellar mass log M_*(M_⊙) > 10.15 in galaxy clusters with mass M₅₀₀ > 10¹³ M_⊙ extracted from box2b (640 comoving Mpc h⁻¹) of the Magneticum Pathfinder suite of cosmological hydrodynamical simulations at redshifts 0.25 and 0.90. We examined the influence of stellar mass, distance to the nearest neighbouring galaxy, cluster-centric radius, substructure membership, and large-scale surroundings on the fraction of galaxies hosting an AGN, star formation rate, and the ratio between star-forming and quiescent galaxies.

Results. We find that in low-mass galaxies, AGN activity and star formation are similarly affected by the environment and decline towards the cluster centre. In massive galaxies, the impact is different; star-formation level increases in the inner regions and peaks between 0.5 and 1 R₅₀₀ with a rapid decline in the centre, whereas AGN activity declines in the inner regions and rapidly rises below R₅₀₀ towards the centre. We suggest that this increase is a result of the larger black hole masses relative to stellar masses in the cluster centre. After disentangling the contributions of neighbouring cluster regions, we find an excess of AGN activity in massive galaxies on the cluster outskirts (∼3 R₅₀₀). We also find that the local density, substructure membership, and stellar mass strongly influence star formation and AGN activity but verify that they cannot fully account for the observed radial trends.