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Register a new userDissecting and Modelling Galaxy Assembly Bias
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Understanding the galaxy-halo connection is fundamental for contemporary models of galaxy clustering. The extent to which the haloes assembly history and environment impact galaxy clustering (a.k.a. galaxy assembly bias; GAB), remains a complex and challenging problem. Using a semi-analytic galaxy formation model, we study the individual contributions of different secondary halo properties to the GAB signal. These are obtained by comparing the clustering of stellar-mass selected samples to that of shuffled samples where the galaxies are randomly reassigned to haloes of fixed mass and a specified secondary halo property. We explore a large range of internal halo properties and environmental measures. We find that commonly used properties like halo age or concentration amount to only 20-30 per cent of the signal, while the smoothed matter density or the tidal anisotropy can account for the full level of GAB (though care should be given to the specific definition). For the successful measures, we examine the occupancy variations and the associated changes in the halo occupation function parameters. These are used to create mock catalogues that reproduce the full level of GAB. Finally, we propose a practical modification of the standard halo occupation distribution model, which can be tuned to any level of assembly bias. Fitting the parameters to our semi-analytic model, we demonstrate that the corresponding mock catalogue recovers the target level of GAB as well as the occupancy variations. Our results enable producing realistic mock catalogues and directly inform theoretical modelling of assembly bias and attempts to detect it in the Universe.
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We use the {sc Illustris TNG300} magneto-hydrodynamic simulation, the {sc SAGE} semi-analytical model, and the subhalo abundance matching technique (SHAM) to examine the diversity in predictions for galaxy assembly bias (i.e. the difference in the large scale clustering of galaxies at a fixed halo mass due to correlations with the assembly history and other properties of host haloes). We consider samples of galaxies selected according to their stellar mass or star formation rate at various redshifts. We find that all models predict an assembly bias signal of different magnitude, redshift evolution, and dependence with selection criteria and number density. To model these non-trivial dependences, we propose an extension to the standard SHAM technique so it can include arbitrary amounts of assembly bias. We do this by preferentially selecting subhaloes with the same internal property but different {it individual} large-scale bias. We find that with this model, we can successfully reproduce the galaxy assembly bias signal in either {sc SAGE} or the {sc TNG}, for all redshifts and galaxy number densities. We anticipate that this model can be used to constrain the level of assembly bias in observations and aid in the creation of more realistic mock galaxy catalogues.
We analyze the spectra of 300,000 luminous red galaxies (LRGs) with stellar masses $M_* gtrsim 10^{11} M_{odot}$ from the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). By studying their star-formation histories, we find two main evolutionary paths converging into the same quiescent galaxy population at $zsim0.55$. Fast-growing LRGs assemble $80%$ of their stellar mass very early on ($zsim5$), whereas slow-growing LRGs reach the same evolutionary state at $zsim1.5$. Further investigation reveals that their clustering properties on scales of $sim$1-30 Mpc are, at a high level of significance, also different. Fast-growing LRGs are found to be more strongly clustered and reside in overall denser large-scale structure environments than slow-growing systems, for a given stellar-mass threshold. Our results imply a dependence of clustering on stellar-mass assembly history (naturally connected to the mass-formation history of the corresponding halos) for a homogeneous population of similar mass and color, which constitutes a strong observational evidence of galaxy assembly bias.
We examine the evolution of assembly bias using a semi-analytical model of galaxy formation implemented in the Millennium-WMAP7 N-body simulation. We consider fixed number density galaxy samples ranked by stellar mass or star formation rate. We investigate how the clustering of haloes and their galaxy content depend on halo formation time and concentration, and how these relationships evolve with redshift. At $z=0$ the dependences of halo clustering on halo concentration and formation time are similar. However, at higher redshift, halo assembly bias weakens for haloes selected by age, and reverses and increases for haloes selected by concentration. The variation of the halo occupation with concentration and formation time is also similar at $z=0$ and changes at higher redshifts. In this case, the occupancy variation with halo age stays mostly constant with redshift but decreases for concentration. Finally, we look at the evolution of assembly bias reflected in the galaxy distribution by examining the galaxy correlation functions relative to those of shuffled galaxy samples which remove the occupancy variation. This correlation functions ratio monotonically decreases with larger redshift and for lower number density samples, going below unity in some cases, leading to reduced galaxy clustering. While the halo occupation functions themselves vary, the assembly bias trends are similar whether selecting galaxies by stellar mass or star formation rate. Our results provide further insight into the origin and evolution of assembly bias. Our extensive occupation function measurements and fits are publicly available and can be used to create realistic mock catalogues.
Halo assembly bias is the secondary dependence of the clustering of dark-matter haloes on their assembly histories at fixed halo mass. This established dependence is expected to manifest itself on the clustering of the galaxy population, a potential effect commonly known as galaxy assembly bias. Using the IllustrisTNG300 magnetohydrodynamical simulation, we analyse the dependence of the properties and clustering of galaxies on the shape of the specific mass accretion history of their hosting haloes (sMAH). We first show that several halo and galaxy properties strongly correlate with the slope of the sMAH ($beta$) at fixed halo mass. Namely, haloes with increasingly steeper $beta$ increment their halo masses faster at early times, and their hosted galaxies present larger stellar-to-halo mass ratios, lose their gas faster, reach the peak of their star formation histories at higher redshift, and become quenched earlier. We also demonstrate that $beta$ is more directly connected to these key galaxy formation properties than other broadly employed halo proxies, such as formation time. Finally, we measure the secondary dependence of galaxy clustering on $beta$ at fixed halo mass as a function of redshift. By tracing back the evolution of individual haloes, we show that the amplitude of the galaxy assembly bias signal for the progenitors of $z=0$ galaxies increases with redshift, reaching a factor of 2 at $z = 1$ for haloes of $M_mathrm{halo}=10^{11.5}-10^{12}$ $h^{-1}mathrm{M}_odot$. The measurement of the evolution of assembly bias along the merger tree provides a new theoretical perspective to the study of secondary bias. Our findings, which show a tight relationship between halo accretion and both the clustering and the observational properties of the galaxy population, have also important implications for the generation of mock catalogues for upcoming cosmological surveys.
We study the dependence of the galaxy content of dark matter halos on large-scale environment and halo formation time using semi-analytic galaxy models applied to the Millennium simulation. We analyze subsamples of halos at the extremes of these distributions and measure the occupation functions for the galaxies they host. We find distinct differences in these occupation functions. The main effect with environment is that central galaxies (and in one model also the satellites) in denser regions start populating lower-mass halos. A similar, but significantly stronger, trend exists with halo age, where early-forming halos are more likely to host central galaxies at lower halo mass. We discuss the origin of these trends and the connection to the stellar mass -- halo mass relation. We find that, at fixed halo mass, older halos and to some extent also halos in dense environments tend to host more massive galaxies. Additionally, we see a reverse trend for the satellite galaxies occupation where early-forming halos have fewer satellites, likely due to having more time for them to merge with the central galaxy. We describe these occupancy variations also in terms of the changes in the occupation function parameters, which can aid in constructing realistic mock galaxy catalogs. Finally, we study the corresponding galaxy auto- and cross-correlation functions of the different samples and elucidate the impact of assembly bias on galaxy clustering. Our results can inform theoretical models of assembly bias and attempts to detect it in the real universe.
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