Limitations to the "basic'' HOD model and beyond

TL;DR

This study uses the IllustrisTNG simulations to evaluate and improve the standard Halo Occupation Distribution model by identifying its limitations and incorporating secondary halo properties to better match galaxy clustering data.

Contribution

The paper demonstrates the shortcomings of the basic HOD model and introduces environmental and secondary parameters to enhance its accuracy in predicting galaxy clustering.

Findings

Basic HOD underpredicts galaxy clustering by ~15% on certain scales.

Including environment and secondary halo properties improves HOD predictions.

Secondary parameters like local environment and velocity anisotropy are effective in modeling assembly bias.

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, we find that, on average, the ``basic'' HOD model underpredicts the real-space correlation function in the TNG300 box by $\sim$ 15\% on scales of $1 \ {\rm Mpc}/h < r < 20 \ {\rm Mpc}/h$, 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 ``basic'' HOD, we find that the local environment of the halo, the velocity dispersion anisotropy, $\beta$, and the product of the half-mass radius and the velocity dispersion, $\sigma^2 R_{\rm halfmass}$, are the three most effective measures of assembly bias that help reconcile the ``basic'' HOD-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 we combine both mass and local environment information about the halo.