Galaxy assembly bias in cosmological hydrodynamical simulations: a comparison between SIMBA and IllustrisTNG

TL;DR

This work investigates galaxy assembly bias (GAB) using two state-of-the-art hydrodynamical simulations, SIMBA and IllustrisTNG, to understand how baryonic physics and cosmic environment shape galaxy clustering beyond halo mass. By applying the shuffling technique and examining environmental indicators δ_5 (overdensity) and α_5 (tidal anisotropy), the authors show that GAB strengthens from $z=5$ to $z=2$ (to about $5 ight)%$ in both simulations but then diverges toward $z=0$: SIMBA returns to near-zero GAB while TNG reaches ~$10 ight)%$. They demonstrate that most of the GAB originates from low-mass haloes and that δ_5 largely drives the effect, with α_5 contributing negligibly in SIMBA and only modestly in TNG. The halo occupation distribution (HOD) analyses reveal density-dependent shifts primarily for low-mass halos, enabling a practical parameterization of δ_5-driven GAB for improved HOD-based mock catalogues. These results highlight the sensitivity of GAB to baryonic physics and provide a pathway to incorporate environment-dependent corrections into cosmological analyses of galaxy surveys. $w_p(r_p)$, environmental metrics, and HOD fits are central to these conclusions.

Abstract

Modelling of large-scale structure is increasingly concerned with galaxy assembly bias (GAB), the dependence of galaxy clustering on quantities other than halo mass. We investigate how baryonic physics affects the strength and redshift evolution of GAB using the largest runs of two state-of-the-art cosmological hydrodynamical simulations: SIMBA and IllustrisTNG. We quantify GAB by comparing the clustering of stellar-mass-selected galaxies to that of shuffled samples, where galaxies are randomly reassigned to haloes of similar mass. We find that GAB in both simulations increases from approximately zero at $z=5$ to a $\sim\,5\%$ change in clustering amplitude at $z=2$. After this epoch, the trends diverge: GAB in TNG continues to increase, reaching $\sim\,10\%$ at $z=0$, while in SIMBA it decreases to nearly zero. By further shuffling galaxies within bins of halo mass and cosmic environment -- characterised by smoothed matter overdensity ($δ_5$) and tidal anisotropy ($α_5$) -- we show that most of the GAB in both simulations can be attributed to the overdensity, while tidal anisotropy contributes negligibly in both simulations. Exploring this effect from the point of view of the halo occupation distribution (HOD), we find that numbers of central and satellite galaxies vary with overdensity -- but only near the respective turn-on masses for these two constituents: the galaxy contents of high-mass haloes are very nearly independent of environment. We present a simple parameterisation that allows the HOD modelling to be modified to reflect this form of density-dependent GAB.

Figures (12)

• Figure 1: The effect of galaxy assembly bias on galaxy clustering, as quantified by $[w_{\mathrm{p}}(r_{\mathrm{p}})/w_{\mathrm{p}}(r_{\mathrm{p}})_{\mathrm{shuffled}}-1]$ at redshifts $z=5$, $4$, $3$, $2$, $1$, and $0$. The left panel shows the results for SIMBA; the right panel for TNG. Error bars indicate $1\sigma$ uncertainties computed using Eq. (\ref{['error']}).
• Figure 2: The distribution of haloes on the $\delta_5-\alpha_5$ plane. Each orange (blue) dots is one halo from the SIMBA (TNG) sample at $z=0$. The contours indicate the 2-dimensional probability distribution of galaxies on the $\delta_5-\alpha_5$ plane. The marginalized distributions for $\delta_5$ and $\alpha_5$ are shown in the top and right panels, respectively.
• Figure 3: Comparison between the total GAB (dashed lines; shuffled by halo mass only) and the GAB with cosmic environment dependence removed (solid lines; shuffled by both halo mass and [$\delta_5$ or $\alpha_5$] ) at $z=2,1,0$. Left and right panels show results from SIMBA and TNG, respectively. Top and bottom panels correspond to cosmic environment defined by $\delta_5$ and $\alpha_5$, respectively. The difference between dashed and solid curves reflects the contribution of the corresponding cosmic environment variable to the GAB.
• Figure 4: HOD curves measured from SIMBA at $z=0$ (left) and $z=2$ (right). The total HOD of all galaxies for SIMBA is shown as the brown solid line. The total HOD is decomposed into contributions from central galaxies (gray line) and satellite galaxies (black line). Solid lines represent measurements from all haloes in the sample, while dashed (dotted) lines correspond to haloes in the highest (lowest) 25% of the cosmic environment distribution, quantified by $\delta_5$ in the corresponding halo mass bin. In each panel we present the ratio of each subsample’s HOD to the corresponding total HOD at the bottom, highlighting deviations from the mean.
• Figure 5: The same as Fig. \ref{['simba_delta']}, but for TNG.