Limitations to the ‘basic’ HOD model and beyond

Abstract

We make use of the IllustrisTNG cosmological, hydrodynamical simulations to test fundamental assumptions of the mass-based Halo Occupation Distribution (HOD) approach to modelling the galaxy-halo connection. By comparing the clustering of galaxies measured in the 300 Mpc TNG box (TNG300) with that predicted by the standard ("basic") HOD model (bHOD), we find that, on average, the bHOD model underpredicts the real-space correlation function in the TNG300 box by ∼ 15% on scales of 1 Mpc/h < r < 20 Mpc/h, which is well beyond the target precision demanded of next-generation galaxy redshift surveys. We perform several tests to establish the robustness of our findings to systematic effects, including the effect of finite box size and the choice of halo finder. In our exploration of "secondary" parameters with which to augment the bHOD, we find that the local environment of the halo, the velocity dispersion anisotropy, β, and the product of the half-mass radius and the velocity dispersion, σ 2 R halfmass , are the three most effective measures of assembly bias that help reconcile the bHOD-predicted clustering with that in TNG300. In addition, we test other halo properties such as halo spin, formation epoch and halo concentration. We also find that at fixed halo mass, galaxies in one type of environment cluster differently from galaxies in another. We demonstrate that a more complete model of the galaxy-halo connection can be constructed if both the mass and information regarding the local environment in which a halo is embedded are combined.

simba: the average properties of the circumgalactic medium of 2 ≤ z ≤ 3 quasars are determined primarily by stellar feedback

“…As such, our technique has a stronger physical motivation, as real QSOs are characterised by a high BH accretion rate and strong clustering. Thus, unlike in Sorini et al (2018), our method provides a good match to the observed QSO clustering properties (White et al 2012) without assuming a purely mass-based HOD, which can be simplistic (Beltz-Mohrmann et al 2020;Hadzhiyska et al 2020). At the same time, the number of haloes that we select is the same that we would have selected if we had simply considered all haloes with mass above M min .…”

Galaxy assembly bias and large-scale distribution: a comparison between IllustrisTNG and a semi-analytic model

“…In previous works, we found that conditioning on this parameter in TNG leads to a substantial increase in the galaxy clustering on large scales (Hadzhiyska et al 2020(Hadzhiyska et al , 2021a. We conjectured that at a fixed mass, haloes residing in denser regions contain more galaxies on average than haloes in underdense regions due to experiencing more mergers and having a larger gas reservoir with which to form stars (Abbas & Sheth 2007;Pujol et al 2017;Paranjape, Hahn & Sheth 2018;Shi & Sheth 2018).…”

“…In this section, we review the definitions of the various halo parameters employed in this study (for more details, see Hadzhiyska et al 2020). Appendix B discusses the correlations between the various parameters in addition to the halo assembly bias measured for concentration and environment.…”

“…Unfortunately, the simplest versions of such empirical approaches have been shown to make predictions that have significant discrepancies with observations and more detailed galaxy formation models (Croton, Gao & White 2007;Paranjape et al 2015;Beltz-Mohrmann, Berlind & Szewciw 2020). For example, several recent works have argued that including secondary HOD parameters, such as the local halo environment and concentration, yields better agreement with hydrodynamical simulations as compared to the standard prescription (Hadzhiyska et al 2020;Xu & Zheng 2020;Hadzhiyska et al 2021a). Related approaches include Sub-Halo Abundance Matching models (SHAMs) and semiempirical models, which derive mappings between galaxy and DM halo properties such that specific observational quantities are matched (Behroozi, Wechsler & Conroy 2013b;Moster, Naab & White 2013Tacchella, Trenti & Carollo 2013;Tacchella et al 2018;Behroozi et al 2019).…”

“…These sub-grid processes are typically phenomenological, and contain adjustable parameters which are typically calibrated to reproduce a set of observations. Having recently reached sizes of several hundred megaparsecs on a side (Chaves-Montero et al 2016;Springel et al 2018), hydro simulations now allow us to test the accuracy of galaxy population models by comparing various statistical properties of the galaxy field side by side (Bose et al 2019;Hadzhiyska et al 2020;Contreras, Angulo & Zennaro 2021). Such analyses have been performed extensively for galaxy populations above a certain stellar mass cut (as stellar-mass cuts are a good proxy for luminosity cuts), and a few studies have focused on other galaxy populations targeted by future surveys, such as star-forming emission-line galaxies (ELGs) (e.g.…”