From ASTRID to BRAHMA – The role of overmassive black holes in little red dots in cosmological simulations
8 affiliations
- 1Carnegie Mellon University
- 2University of Southampton
- 3University of Virginia
- 4Center for Astrophysics Harvard & Smithsonian
- 5University of Florida
- 6University of Michigan
- 7Institute for Advanced Study
- 8University of California, Riverside
Abstract
We investigate the origin of Little Red Dots (LRDs) using a realization of the BRAHMA cosmological hydrodynamic simulation that produces overmassive black holes at z ∼ 4 - 8 . We generate mock JWST observations by modeling galaxy spectral energy distributions (including dust attenuation) together with AGN emission using cloudy-based templates, and test whether such systems reproduce the observed LRD population. We consider both a standard low-redshift AGN emission model and a gas-enshrouded AGN emission model, in which the AGN is embedded within unresolved dense gas that produces a strong Balmer break, and find that LRDs emerge in significantly larger numbers with the latter model. Assuming that all AGN reside in this gas-enshrouded phase, we recover LRD number densities ( Φ ∼ 2 × 10 − 4 M p c − 3 ), close to the observed abundance. This implies an effective gas-enshrouded duty cycle approaching unity, which is not predicted by current theoretical models. Current JWST constraints on the high-z black hole mass function suggest that this tension may arise from a deficit of low-mass () LRDs in BRAHMA. Applying the same gas-enshrouded model to the ASTRID simulation, which lacks overmassive black holes, we find that the LRD abundance remains below observations. Overall, while the overmassive black holes in this BRAHMA realization substantially enhance the production of LRD-like systems, reproducing the observed LRD abundance requires them to reside in dense gas environments with unrealistically high duty cycles. We therefore conclude that overmassive black holes are unlikely to be the sole origin of the observed JWST LRD population, and may instead account for only a subset of these systems.
Related
- Exploring the observed properties of JWST Little Red Dots with cosmological simulations and dust radiative transfer
- Halo-driven Origin and Evolution of Overmassive Black Holes and Little Red Dots
- A Spectral Framework for Testing the Quasi-Star Hypothesis in Little Red Dots I: Weighing LRDs by Their Super-Eddington Luminosity Ratios---No Signs of Overmassive Black Holes
- Overmassive No More: The Case for Little Red Dots Hosting Black Hole Seeds as Massive as Single Supermassive Stars
- A Cosmological Framework for Stellar Collisions at High Redshift in Proto-globular Clusters, Nuclear Star Clusters, and Little Red Dots
BibTeX
@article{lachance2026from,
title = {{From ASTRID to BRAHMA – The role of overmassive black holes in little red dots in cosmological simulations}},
author = {Patrick LaChance and Aklant Kumar Bhowmick and Rupert A. C. Croft and Tiziana Di Matteo and Yihao Zhou and Fabio Pacucci and Laura Blecha and Paul Torrey and Yueying Ni and Nianyi Chen and Simeon Bird},
journal = {The Open Journal of Astrophysics},
year = {2026},
month = oct,
doi = {10.33232/001c.173389},
url = {https://astro.theoj.org/article/173389-from-astrid-to-brahma-the-role-of-overmassive-black-holes-in-little-red-dots-in-cosmological-simulations},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - From ASTRID to BRAHMA – The role of overmassive black holes in little red dots in cosmological simulations
AU - Patrick LaChance
AU - Aklant Kumar Bhowmick
AU - Rupert A. C. Croft
AU - Tiziana Di Matteo
AU - Yihao Zhou
AU - Fabio Pacucci
AU - Laura Blecha
AU - Paul Torrey
AU - Yueying Ni
AU - Nianyi Chen
AU - Simeon Bird
PY - 2026
DA - 2026/10/30
JO - The Open Journal of Astrophysics
DO - 10.33232/001c.173389
UR - https://astro.theoj.org/article/173389-from-astrid-to-brahma-the-role-of-overmassive-black-holes-in-little-red-dots-in-cosmological-simulations
AB - We investigate the origin of Little Red Dots (LRDs) using a realization of the BRAHMA cosmological hydrodynamic simulation that produces overmassive black holes at z ∼ 4 - 8 . We generate mock JWST observations by modeling galaxy spectral energy distributions (including dust attenuation) together with AGN emission using cloudy-based templates, and test whether such systems reproduce the observed LRD population. We consider both a standard low-redshift AGN emission model and a gas-enshrouded AGN emission model, in which the AGN is embedded within unresolved dense gas that produces a strong Balmer break, and find that LRDs emerge in significantly larger numbers with the latter model. Assuming that all AGN reside in this gas-enshrouded phase, we recover LRD number densities ( Φ ∼ 2 × 10 − 4 M p c − 3 ), close to the observed abundance. This implies an effective gas-enshrouded duty cycle approaching unity, which is not predicted by current theoretical models. Current JWST constraints on the high-z black hole mass function suggest that this tension may arise from a deficit of low-mass () LRDs in BRAHMA. Applying the same gas-enshrouded model to the ASTRID simulation, which lacks overmassive black holes, we find that the LRD abundance remains below observations. Overall, while the overmassive black holes in this BRAHMA realization substantially enhance the production of LRD-like systems, reproducing the observed LRD abundance requires them to reside in dense gas environments with unrealistically high duty cycles. We therefore conclude that overmassive black holes are unlikely to be the sole origin of the observed JWST LRD population, and may instead account for only a subset of these systems.
SN - 2565-6120
ER -