Early Supermassive Black Holes and Little Red Dots Require Free-Fall Growth
Abstract
Supermassive black holes/active galactic nuclei (SMBHs/AGN), forming only a few hundred million years after the Big Bang as observed with the James Webb Space Telescope (JWST), challenge theoretical understanding. How could they grow so massive (MBH > 106 M⊙) so quickly after initial seeding? Is this rapid growth related to the numerous and enigmatic Little Red Dots (LRDs), compact sources with AGN-like characteristics, discovered by JWST? To address these mysteries, we consider the first-order constraint on SMBH growth: enough baryonic material has to reach the vicinity of the SMBH seed, located near the bottom of the gravitational potential well of the host dark matter halo. We specifically examine cold-mode accretion, where gas from the cosmic environment flows into the virialized halo in cold streams without being shock-heated, efficiently reaching the center on a free-fall timescale. We find that cold mode accretion is necessary to supply material for the SMBHs to reach the observed masses, whereas for shock-heated gas inflow the required amount could only be supplied by implausibly rare halos. Moreover, cold-mode inflow in rare (∼1 Gpc−3) halos matches the mass and number of the massive quasars, and halos able to support super-Eddington accretion for massive SMBHs (∼107 M⊙) match LRD number densities. The decreasing LRD abundance at lower redshifts may then reflect the termination of cold-mode accretion in the growing host halos. The populations of massive SMBHs and LRDs at early times may thus arise naturally from cosmological structure formation, based on the abundance of halos capable of supplying sufficient material through cold accretion.
Related
- Heavy Seeds and the Multi-scale Fueling of Early Supermassive Black Holes and Little Red Dots
- Chaotic accretion can (still) explain early supermassive black hole growth
- Chaotic accretion can (still) explain early supermassive black hole growth
- Inflated Supermassive Stars as Little Red Dots and Progenitors of Supermassive Black Holes
- Overmassive No More: The Case for Little Red Dots Hosting Black Hole Seeds as Massive as Single Supermassive Stars
BibTeX
@misc{jeon2026early,
title = {{Early Supermassive Black Holes and Little Red Dots Require Free-Fall Growth}},
author = {Junehyoung Jeon and Volker Bromm and Michael Boylan-Kolchin and Julian B. Muñoz},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.00115},
eprint = {2609.00115},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.00115},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Early Supermassive Black Holes and Little Red Dots Require Free-Fall Growth
AU - Junehyoung Jeon
AU - Volker Bromm
AU - Michael Boylan-Kolchin
AU - Julian B. Muñoz
PY - 2026
DA - 2026/09/01
PB - arXiv
DO - 10.48550/arxiv.2609.00115
UR - https://arxiv.org/abs/2609.00115
AB - Supermassive black holes/active galactic nuclei (SMBHs/AGN), forming only a few hundred million years after the Big Bang as observed with the James Webb Space Telescope (JWST), challenge theoretical understanding. How could they grow so massive (M_BH > 10^6 M_⊙) so quickly after initial seeding? Is this rapid growth related to the numerous and enigmatic Little Red Dots (LRDs), compact sources with AGN-like characteristics, discovered by JWST? To address these mysteries, we consider the first-order constraint on SMBH growth: enough baryonic material has to reach the vicinity of the SMBH seed, located near the bottom of the gravitational potential well of the host dark matter halo. We specifically examine cold-mode accretion, where gas from the cosmic environment flows into the virialized halo in cold streams without being shock-heated, efficiently reaching the center on a free-fall timescale. We find that cold mode accretion is necessary to supply material for the SMBHs to reach the observed masses, whereas for shock-heated gas inflow the required amount could only be supplied by implausibly rare halos. Moreover, cold-mode inflow in rare (∼1 Gpc^−3) halos matches the mass and number of the massive quasars, and halos able to support super-Eddington accretion for massive SMBHs (∼10^7 M_⊙) match LRD number densities. The decreasing LRD abundance at lower redshifts may then reflect the termination of cold-mode accretion in the growing host halos. The populations of massive SMBHs and LRDs at early times may thus arise naturally from cosmological structure formation, based on the abundance of halos capable of supplying sufficient material through cold accretion.
ER -