The impact of assembly bias on the halo occupation in hydrodynamical simulations

TL;DR

This study examines how galaxy occupancy in dark matter haloes varies with environment and formation time using hydrodynamical simulations, revealing that dense environments and early formation favor central galaxy hosting, with implications for galaxy formation models.

Contribution

It provides the first detailed analysis of assembly bias effects on galaxy occupation in hydrodynamical simulations, confirming and extending previous semi-analytical model results.

Findings

Low-mass haloes in dense environments are more likely to host central galaxies.

Early-formed haloes tend to host more central galaxies and fewer satellites.

Dense environment and early formation correlate with more massive central galaxies.

Abstract

We investigate the variations in galaxy occupancy of the dark matter haloes with the large-scale environment and halo formation time, using two state-of-the-art hydrodynamical cosmological simulations, EAGLE and Illustris. For both simulations, we use three galaxy samples with a fixed number density ranked by stellar mass. For these samples we find that low-mass haloes in the most dense environments are more likely to host a central galaxy than those in the least dense environments. When splitting the halo population by formation time, these relations are stronger. Hence, at a fixed low halo mass, early-formed haloes are more likely to host a central galaxy than late-formed haloes since they have had more time to assemble. The satellite occupation shows a reverse trend where early-formed haloes host fewer satellites due to having more time to merge with the central galaxy. We also analyse the stellar mass -- halo mass relation for central galaxies in terms of the large-scale environment and formation time of the haloes. We find that low mass haloes in the most dense environment host relatively more massive central galaxies. This trend is also found when splitting the halo population by age, with early-formed haloes hosting more massive galaxies. Our results are in agreement with previous findings from semi-analytical models, providing robust predictions for the occupancy variation signature in the halo occupation distribution of galaxy formation models.