Mergers may not be the main channel for galaxy stellar mass growth at redshift z≳5: Predictions from cosmological hierarchical models and simulations
Abstract
Mergers are a crucial channel of galaxy growth, but their role relative to star formation remains uncertain at high redshift. Here we investigate merger rates and merger-driven stellar mass assembly for galaxies with stellar mass M⋆ ≳ 108 M⊙ at z≃ 5−11, by combining complementary theoretical approaches within ΛCDM. These include the DECODE semi-empirical model, the TNG100 and EAGLE hydrodynamical simulations, and the \textsc{L-Galaxies} semi-analytical model, together with the Millennium and TNG100-Dark simulations used to examine dark matter subhalo statistics and merger timescales. This compares models with different treatments of baryonic physics and galaxy-halo connections. We find that dark matter-only simulations predict merger timescales ∼ 30% longer than those inferred from cosmological simulations including baryons. Galaxy major merger (M⋆,sat / M⋆,cen > 0.25) rates are high at cosmic dawn and increase towards earlier epochs, reaching ∼ 1/Gyr at z∼ 5 and peaking at ∼ 3/Gyr at z∼ 8. Despite these high rates, mergers contribute only ≲ 10% of the total cosmic stellar mass density evolution. The ex-situ stellar mass fraction ranges from a few percent to ∼ 20% for all galaxies at z≳ 5, across different models and simulations. Their broad agreement implies that stellar mass assembly at cosmic dawn is dominated by in-situ star formation rather than mergers. These findings suggest that the high pair fractions identified by JWST may not directly trace efficient ex-situ stellar mass growth, highlighting the need for physically motivated merger timescales to interpret galaxy assembly in the early Universe.
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BibTeX
@misc{fu2026mergers,
title = {{Mergers may not be the main channel for galaxy stellar mass growth at redshift $z\gtrsim5$: Predictions from cosmological hierarchical models and simulations}},
author = {Hao Fu and Mohammadreza Ayromlou and Francesco Shankar and Lars Hernquist and Andrea Lapi and Daniel Roberts and Feng Yuan and Maulik Bhatt and Evgenii Chaikin and Simona Gallerani and Luca Graziani and Antonio La Marca and George Mountrichas and Yingjie Peng and Haocheng Wang and Kai Wang},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.17575},
eprint = {2610.17575},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.17575},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Mergers may not be the main channel for galaxy stellar mass growth at redshift z≳5: Predictions from cosmological hierarchical models and simulations
AU - Hao Fu
AU - Mohammadreza Ayromlou
AU - Francesco Shankar
AU - Lars Hernquist
AU - Andrea Lapi
AU - Daniel Roberts
AU - Feng Yuan
AU - Maulik Bhatt
AU - Evgenii Chaikin
AU - Simona Gallerani
AU - Luca Graziani
AU - Antonio La Marca
AU - George Mountrichas
AU - Yingjie Peng
AU - Haocheng Wang
AU - Kai Wang
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.17575
UR - https://arxiv.org/abs/2610.17575
AB - Mergers are a crucial channel of galaxy growth, but their role relative to star formation remains uncertain at high redshift. Here we investigate merger rates and merger-driven stellar mass assembly for galaxies with stellar mass M_⋆ ≳ 10^8 M_⊙ at z≃ 5−11, by combining complementary theoretical approaches within ΛCDM. These include the DECODE semi-empirical model, the TNG100 and EAGLE hydrodynamical simulations, and the \textsc{L-Galaxies} semi-analytical model, together with the Millennium and TNG100-Dark simulations used to examine dark matter subhalo statistics and merger timescales. This compares models with different treatments of baryonic physics and galaxy-halo connections. We find that dark matter-only simulations predict merger timescales ∼ 30% longer than those inferred from cosmological simulations including baryons. Galaxy major merger (M_⋆,sat / M_⋆,cen > 0.25) rates are high at cosmic dawn and increase towards earlier epochs, reaching ∼ 1/Gyr at z∼ 5 and peaking at ∼ 3/Gyr at z∼ 8. Despite these high rates, mergers contribute only ≲ 10% of the total cosmic stellar mass density evolution. The ex-situ stellar mass fraction ranges from a few percent to ∼ 20% for all galaxies at z≳ 5, across different models and simulations. Their broad agreement implies that stellar mass assembly at cosmic dawn is dominated by in-situ star formation rather than mergers. These findings suggest that the high pair fractions identified by JWST may not directly trace efficient ex-situ stellar mass growth, highlighting the need for physically motivated merger timescales to interpret galaxy assembly in the early Universe.
ER -