The causal effect of environment on halo mass and concentration

TL;DR

This paper introduces a novel resimulation method to isolate the causal effects of environment on dark matter halo properties, revealing that halo mass is mainly influenced by internal density, while concentration depends more on surroundings.

Contribution

The authors develop a new splicing technique for resimulating individual halos in varied environments, enabling causal analysis of environmental impacts on halo characteristics.

Findings

Halo mass is primarily determined by internal density structure.

Halo concentration is more strongly affected by the large-scale environment.

The method allows for studying environmental effects on galaxy evolution processes.

Abstract

Understanding the impact of environment on the formation and evolution of dark matter halos and galaxies is a crucial open problem. Studying statistical correlations in large simulated populations sheds some light on these impacts, but the causal effect of an environment on individual objects is harder to pinpoint. Addressing this, we present a new method for resimulating a single dark matter halo in multiple large-scale environments. In the initial conditions, we 'splice' (i.e. insert) the Lagrangian region of a halo into different Gaussian random fields, while enforcing consistency with the statistical properties of $\Lambda$CDM. Applying this technique, we demonstrate that the mass of halos is primarily determined by the density structure inside their Lagrangian patches, while the halos' concentration is more strongly affected by environment. The splicing approach will also allow us to study, for example, the impact of the cosmic web on accretion processes and galaxy quenching.