# Mapping stellar content to dark matter halos - III.Environmental dependence and conformity of galaxy colours

## TL;DR

This study shows that galaxy colour conformity and environmental dependence can be explained by halo quenching and halo mass function variation without invoking galaxy assembly bias, aligning well with SDSS observations.

## Contribution

The paper demonstrates that a halo mass quenching model without galaxy assembly bias can explain galaxy colour conformity and environmental effects, challenging previous assumptions.

## Key findings

- Halo quenching explains galaxy colour conformity.
- Environmental dependence arises from halo mass function variation.
- Models with galaxy assembly bias struggle to match all observables.

## Abstract

Recent studies suggest that the quenching properties of galaxies are correlated over several mega-parsecs. The large-scale "galactic conformity" phenomenon around central galaxies has been regarded as a potential signature of "galaxy assembly bias" or "pre-heating", both of which interpret conformity as a result of direct environmental effects acting on galaxy formation. Building on the iHOD halo quenching framework developed in Zu & Mandelbaum (2015, 2016), we discover that our fiducial halo mass quenching model, without any galaxy assembly bias, can successfully explain the overall environmental dependence and the conformity of galaxy colours in SDSS, as measured by the mark correlation functions of galaxy colours and the red galaxy fractions around isolated primaries, respectively. Our fiducial iHOD halo quenching mock also correctly predicts the differences in the spatial clustering and galaxy-galaxy lensing signals between the more vs. less red galaxy subsamples, split by the red-sequence ridge-line at fixed stellar mass. Meanwhile, models that tie galaxy colours fully or partially to halo assembly bias have difficulties in matching all these observables simultaneously. Therefore, we demonstrate that the observed environmental dependence of galaxy colours can be naturally explained by the combination of 1) halo quenching and 2) the variation of halo mass function with environment --- an indirect environmental effect mediated by two separate physical processes.