Don't Forget the Forest for the Trees: The Stellar-Mass Halo-Mass Relation in Different Environments

TL;DR

This study uses high-resolution simulations to show that galaxy stellar-to-halo mass ratios depend on large-scale environment, challenging the common assumption that halo mass alone determines galaxy properties.

Contribution

It demonstrates that environment influences galaxy growth, revealing the importance of considering large-scale structure in galaxy-halo connection models.

Findings

Stellar mass to halo mass ratio is higher in overdense regions for certain halo masses.

Assembly bias affects galaxy properties, with earlier formation in overdense environments.

Environmental effects persist even when local density is controlled.

Abstract

The connection between dark matter halos and galactic baryons is often not well-constrained nor well-resolved in cosmological hydrodynamical simulations. Thus, Halo Occupation Distribution (HOD) models that assign galaxies to halos based on halo mass are frequently used to interpret clustering observations, even though it is well-known that the assembly history of dark matter halos is related to their clustering. In this paper we use high-resolution hydrodynamical cosmological simulations to compare the halo and stellar mass growth of galaxies in a large-scale overdensity to those in a large-scale underdensity (on scales of about 20 Mpc). The simulation reproduces assembly bias, that halos have earlier formation times in overdense environments than in underdense regions. We find that the stellar mass to halo mass ratio is larger in overdense regions in central galaxies residing in halos with masses between 10$^{11}$-10$^{12.9}$ M$_{\odot}$. When we force the local density (within 2 Mpc) at z=0 to be the same for galaxies in the large-scale over- and underdensities, we find the same results. We posit that this difference can be explained by a combination of earlier formation times, more interactions at early times with neighbors, and more filaments feeding galaxies in overdense regions. This result puts the standard practice of assigning stellar mass to halos based only on their mass, rather than considering their larger environment, into question.