What determines large scale galaxy clustering: halo mass or local density?

TL;DR

This study demonstrates that local density, rather than halo mass, primarily determines large-scale galaxy clustering, challenging standard models and suggesting the inclusion of local density in galaxy bias predictions.

Contribution

The paper shows that local density, not halo mass, governs halo bias and galaxy clustering, leading to improved HOD models incorporating local density.

Findings

Halo bias correlates with local density, not halo mass.

Standard HOD models fail to predict galaxy bias accurately.

Incorporating local density improves HOD predictions.

Abstract

Using dark matter simulations we show how halo bias is determined by local density and not by halo mass. This is not totally surprising, as according to the peak-background split model, local density is the property that constraints bias at large scales. Massive haloes have a high clustering because they reside in high density regions. Small haloes can be found in a wide range of environments which determine their clustering amplitudes differently. This contradicts the assumption of standard Halo Occupation Distribution (HOD) models that the bias and occupation of haloes is determined solely by their mass. We show that the bias of central galaxies from semi-analytic models of galaxy formation as a function of luminosity and colour is not correctly predicted by the standard HOD model. Using local density instead of halo mass the HOD model correctly predicts galaxy bias. These results indicate the need to include information about local density and not only mass in order to correctly apply HOD analysis in these galaxy samples. This new model can be readily applied to observations and has the advantage that the galaxy density can be directly observed, in contrast with the dark matter halo mass.