How are galaxies assigned to halos? Searching for assembly bias in the SDSS galaxy clustering

TL;DR

This study investigates whether galaxy clustering depends on halo properties beyond mass, specifically halo concentration, using SDSS data and simulations, and finds limited evidence for assembly bias affecting galaxy occupation, especially in brighter samples.

Contribution

The paper provides constraints on halo occupation distribution models with assembly bias using SDSS and simulation data, showing limited correlation between halo concentration and galaxy occupation.

Findings

Satellite occupation not correlated with halo concentration at fixed mass.

Modest correlation between central occupation and halo concentration in less luminous samples.

Standard mass-only HOD models are generally preferred over models with assembly bias.

Abstract

Clustering of dark matter halos has been shown to depend on halo properties beyond mass such as halo concentration, a phenomenon referred to as halo assembly bias. Standard halo occupation models (HOD) in large scale structure studies assume that halo mass alone is sufficient in characterizing the connection between galaxies and halos. Modeling of galaxy clustering can face systematic effects if the number of galaxies within a halo is correlated with other halo properties. Using the Small MultiDark-Planck high resolution $N$-body simulation and the clustering measurements of the Sloan Digital Sky Survey (SDSS) DR7 main galaxy sample, we investigate the extent to which the concentration-dependence of halo occupation can be constrained. Furthermore, we study how allowing for the concentration dependence can improve our modeling of galaxy clustering. Our constraints on HOD with assembly bias suggest that satellite population is not correlated with halo concentration at fixed halo mass. At fixed halo mass, our constraints favor lack of correlation between the occupation of centrals and halo concentration in the most luminous samples ($M_{\rm r}<-21.5,-21$), and modest correlation in the $M_{\rm r}<-20.5,-20, -19.5$ samples. We show that in comparison with abundance-matching mock catalogs, our findings suggest qualitatively similar but modest levels of the impact of halo assembly bias on galaxy clustering. The effect is only present in the central occupation and becomes less significant in brighter galaxy samples. Furthermore, by performing model comparison based on information criteria, we find that in most cases, the standard mass-only HOD model is still favored by the observations.