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

Wang, Yikun; Zehavi, Idit; Contreras, Sergio; Cole, Shaun; Norberg, Peder

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 magnetohydrodynamical simulation TNG300 to investigate how assembly bias affects the environment-dependent galaxy luminosity function (LF). We measure the LFs 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 ∼10% 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 Mr−5logh=−18.5 , which accounts for a ∼20% 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 2. 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.

Citation

Wang, Y., Zehavi, I., Contreras, S., Cole, S., & Norberg, P. (2025). A New Measure of Assembly Bias Using the Environment Dependence of the Luminosity Function. The Astrophysical Journal, 988(2), 280. https://doi.org/10.3847/1538-4357/ade98b

• Journal Article Type: Article
• Acceptance Date: Jun 7, 2025
• Online Publication Date: Jul 30, 2025
• Publication Date: Aug 1, 2025
• Deposit Date: Aug 21, 2025
• Publicly Available Date: Aug 21, 2025
• Journal: The Astrophysical Journal
• Electronic ISSN: 1538-4357
• Peer Reviewed: Peer Reviewed
• Volume: 988
• Issue: 2
• Article Number: 280
• DOI: https://doi.org/10.3847/1538-4357/ade98b
• Keywords: Luminosity function, Galaxies, Galaxy formation, Galaxy dark matter halos, Galaxy luminosities, Magnetohydrodynamical simulations, Hydrodynamical simulations, Dark matter, Large-scale structure of the universe, Galaxy evolution, Observational cosmology
• Public URL: https://durham-repository.worktribe.com/output/4311103