nIFTy Galaxy Cluster simulations VI: The dynamical imprint of substructure on gaseous cluster outskirts

TL;DR

This study uses cosmological hydrodynamical simulations to investigate how accreting substructure influences shock formation and thermodynamics in galaxy cluster outskirts, revealing transient shocks linked to substructure passage.

Contribution

It demonstrates the connection between substructure dynamics and shock features in the ICM, highlighting the consistency across different simulation codes and the potential for observational mapping.

Findings

Strong, transient shocks are driven by substructure passage.

Simulation codes agree on shock timing and location, but differ in shock strength.

Shock structures can serve as indicators of recent cluster merger history.

Abstract

Galaxy cluster outskirts mark the transition region from the mildly non-linear cosmic web to the highly non-linear, virialised, cluster interior. It is in this transition region that the intra-cluster medium (ICM) begins to influence the properties of accreting galaxies and groups, as ram pressure impacts a galaxy's cold gas content and subsequent star formation rate. Conversely, the thermodynamical properties of the ICM in this transition region should also feel the influence of accreting substructure (i.e. galaxies and groups), whose passage can drive shocks. In this paper, we use a suite of cosmological hydrodynamical zoom simulations of a single galaxy cluster, drawn from the nIFTy comparison project, to study how the dynamics of substructure accreted from the cosmic web influences the thermodynamical properties of the ICM in the cluster's outskirts. We demonstrate how features evident in radial profiles of the ICM (e.g. gas density and temperature) can be linked to strong shocks, transient and short-lived in nature, driven by the passage of substructure. The range of astrophysical codes and galaxy formation models in our comparison are broadly consistent in their predictions (e.g. agreeing when and where shocks occur, but differing in how strong shocks will be); this is as we would expect of a process driven by large-scale gravitational dynamics and strong, inefficiently radiating, shocks. This suggests that mapping such shock structures in the ICM in a cluster's outskirts (via e.g. radio synchrotron emission) could provide a complementary measure of its recent merger and accretion history.