Go with the flow: The self-similar and non-linear behaviour of large-scale in- and outflows and the impact of accretion shocks from galaxies to galaxy clusters

¹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians Universität München, Scheinerstr.1, 81679 München, Germany
² Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany

^⋆ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 March 2025
Accepted: 7 July 2025

Abstract

Based on the scale-free nature of gravity, the structure in the Universe is expected to be self-similar on large scales. However, this self-similarity eventually breaks down due to small-scale gas physics such as star formation, active galactic nucleus (AGN) and stellar feedback, and non-linear effects gaining importance relative to linear structure formation. In this work, we investigate the large-scale matter flows that connect collapsed structures to their cosmic environments. Specifically, we focus on their agreement with self-similarity in various properties. For this purpose we used the full power of the hydrodynamical cosmological simulation suite Magneticum Pathfinder to precisely calculate the instantaneous inflow and outflow rates of structures on a large range of masses and redshifts. We find a striking self-similarity across the whole mass range and through time that only breaks down in the outflowing regime due to the different outflow driving mechanisms for galaxies versus galaxy clusters. We additionally performed a geometrical analysis of the patterns of inflow versus outflow to demonstrate how the inflows organise into anisotropic filaments driven by the tidal distortions of the environment, while the outflows are fairly isotropic due to their thermal nature. This also manifests in the differences in the thermal and chemical properties of the gas in the inflowing and outflowing component: While the inflowing gas is pristine and colder, encountering the accretion shock surfaces and entering the influence region of AGN and stellar feedback heats the gas up into a diffuse metal-enriched hot atmosphere. Overall the differences between outflowing and infalling gas are enhanced at the galaxy cluster scale compared to the galaxy scale due to the strong accretion shocks that reach out to large radii for these objects. An individual study of the gas motions in the outskirts of one of the most massive clusters in the simulations we carried out demonstrates these results to greater detail: Gas found in the outer (r > 1.2r_vir) hot atmosphere at z = 0 falls in and is completely enriched early in the assembly process before being shock heated and expanding.