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The MillenniumTNG Project: an improved two-halo model for the galaxy–halo connection of red and blue galaxies

Hadzhiyska, Boryana; Eisenstein, Daniel; Hernquist, Lars; Pakmor, Rüdiger; Bose, Sownak; Delgado, Ana Maria; Contreras, Sergio; Kannan, Rahul; White, Simon D M; Springel, Volker; Frenk, Carlos; Hernández-Aguayo, César; Ferlito, Fulvio; Barrera, Monica

The MillenniumTNG Project: an improved two-halo model for the galaxy–halo connection of red and blue galaxies Thumbnail


Boryana Hadzhiyska

Daniel Eisenstein

Lars Hernquist

Rüdiger Pakmor

Ana Maria Delgado

Sergio Contreras

Rahul Kannan

Simon D M White

Volker Springel

César Hernández-Aguayo

Fulvio Ferlito

Monica Barrera


Approximate methods to populate dark-matter haloes with galaxies are of great utility to galaxy surveys. However, the limitations of simple halo occupation models (HODs) preclude a full use of small-scale galaxy clustering data and call for more sophisticated models. We study two galaxy populations, luminous red galaxies (LRGs) and star-forming emission-line galaxies (ELGs), at two epochs, z = 1 and z = 0, in the large-volume, high-resolution hydrodynamical simulation of the MillenniumTNG project. In a partner study we concentrated on the small-scale, one-halo regime down to r ∼ 0.1 h−1 Mpc, while here we focus on modelling galaxy assembly bias in the two-halo regime, r ≳ 1 h−1 Mpc. Interestingly, the ELG signal exhibits scale dependence out to relatively large scales (r ∼ 20 h−1 Mpc), implying that the linear bias approximation for this tracer is invalid on these scales, contrary to common assumptions. The 10–15 per cent discrepancy is only reconciled when we augment our halo occupation model with a dependence on extrinsic halo properties (‘shear’ being the best-performing one) rather than intrinsic ones (e.g. concentration, peak mass). We argue that this fact constitutes evidence for two-halo galaxy conformity. Including tertiary assembly bias (i.e. a property beyond mass and ‘shear’) is not an essential requirement for reconciling the galaxy assembly bias signal of LRGs, but the combination of external and internal properties is beneficial for recovering ELG the clustering. We find that centrals in low-mass haloes dominate the assembly bias signal of both populations. Finally, we explore the predictions of our model for higher order statistics such as nearest neighbour counts. The latter supplies additional information about galaxy assembly bias and can be used to break degeneracies between halo model parameters.


Hadzhiyska, B., Eisenstein, D., Hernquist, L., Pakmor, R., Bose, S., Delgado, A. M., …Barrera, M. (2023). The MillenniumTNG Project: an improved two-halo model for the galaxy–halo connection of red and blue galaxies. Monthly Notices of the Royal Astronomical Society, 524(2), 2507–2523.

Journal Article Type Article
Acceptance Date Mar 7, 2023
Online Publication Date Jul 20, 2023
Publication Date 2023-09
Deposit Date Feb 6, 2024
Publicly Available Date Feb 6, 2024
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 524
Issue 2
Pages 2507–2523
Public URL


Published Journal Article (2.3 Mb)

Copyright Statement
© 2023 The Author(s)
Published by Oxford University Press on behalf of Royal Astronomical Society

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