Physical processes behind large-scale correlations beyond galactic virial radii

Determine the physical mechanisms that produce correlations between galaxy properties and large-scale environment at distances beyond individual galaxies’ virial radii (often referred to as two-halo galactic conformity), in order to explain why redder galaxies preferentially inhabit overdense regions on multi-megaparsec scales.

Background

Extensive observations show that galaxy properties correlate with environment, including the tendency for redder galaxies to reside in overdense regions and for colors and star formation rates to be synchronized across megaparsec scales (two-halo galactic conformity). While simulations and semi-analytic models can reproduce aspects of these trends, there is no consensus on the dominant physical drivers, with proposed explanations including pre-heating, large-scale tidal effects, and gas removal in filaments or cluster outskirts.

This paper focuses on modeling the dependence of the galaxy–halo connection on large-scale environment using interpretable machine learning (Explainable Boosting Machines) and graph neural networks. Although the study identifies characteristic environmental scales that improve stellar mass predictions, it does not resolve the underlying astrophysical mechanisms responsible for the large-scale correlations, leaving the physical origin of these correlations as an explicit open question.

References

While observations have revealed that redder galaxies preferentially populate overdense environments \citep[e.g.,][]{Dressler1980,Hogg+2003,Blanton+2005}, the physical processes that produce such correlations beyond galaxies' virial radii are not yet well understood \citep[e.g.,][]{2018ARA&A..56..435W,Zehavi+2018}.

— How the Galaxy-Halo Connection Depends on Large-Scale Environment  (2402.07995 - Wu et al., 2024) in Section 1, Introduction

However, their experiment was set up in an idealized way and several open questions connected to the influence of the environment on the CGM persist. Most prominently, the exact physical mechanism that is causing the higher covering fraction in groups is still unclear.

— The environmental dependence of the circumgalactic medium in a high-resolution cosmological simulation  (2609.08654 - Herzog et al., 8 Sep 2026) in Section 1, Introduction