A New Measure of Assembly Bias using the Environment Dependence of the Luminosity Function

TL;DR

This paper introduces a novel, sensitive measure of assembly bias by analyzing the environment dependence of the galaxy luminosity function in simulations, revealing significant environmental effects especially on faint red galaxies.

Contribution

The study demonstrates a new method to quantify assembly bias through the environment-dependent luminosity function, providing insights for observational detection and modeling.

Findings

Assembly bias increases galaxy density in dense environments.

Faint red galaxies show a 20-50% variation in number across environments.

The new measure surpasses previous methods in sensitivity to assembly bias.

Abstract

Assembly bias is the variation in the clustering of dark matter halos and galaxies that arises from correlations between the halo assembly history and the large-scale environment at fixed halo mass. In this work, we use the cosmological magneto-hydrodynamical simulation TNG300 to investigate how assembly bias affects the environment-dependent galaxy luminosity function. We measure the luminosity functions in bins of large-scale environment for the original simulated galaxy sample and for a shuffled sample, where the galaxies are randomly reassigned among halos of similar mass to remove assembly bias. By comparing them, we find distinct signatures, showing variations in the number of galaxies at the $\sim10\%$ level across all luminosities. Assembly bias increases the tendency of galaxies to reside in denser environments and further dilutes underdense regions, beyond the trends governed by halo mass. When separating by color, we see that assembly bias has a much bigger effect on red galaxies fainter than $\mathrm{M_r} - 5 \log{h} = -18.5$, which accounts for a $\sim20\%$ increase in the number of galaxies in the densest environment and a remarkable $50\%$ decrease in the least dense regions. The ratio of these measurements for the densest and least dense regions provides a significant assembly bias signal for the faint red galaxies, larger than a factor of two. Overall, our results provide a novel sensitive measure of assembly bias, offering valuable insight for modeling the effect and a potential new route to detect it in observations.