Early Supermassive Black Holes and Little Red Dots Require Free-Fall Growth

Junehyoung Jeon, Volker Bromm, Michael Boylan-Kolchin, Julian B. Muñoz

First listed 2026-09-02 | Last updated 2026-09-02

Abstract

Short digest

Jeon et al. build an analytic halo-inflow model to ask whether early black holes can be supplied with enough gas at small radii, contrasting ideal cold-stream free fall with shock-heated Bondi-Hoyle inflow. Free-fall cold-mode accretion supplies the observed masses of JWST AGN and high-redshift quasars in plausible halo populations, whereas shock-heated inflow would require implausibly rare hosts. In particular, the rare halos capable of sustaining extreme inflow reproduce massive-quasar abundances, while halos that can fuel super-Eddington growth of roughly 10^7 solar-mass SMBHs align with LRD number densities. The model interprets the declining LRD abundance toward lower redshift as cold-mode accretion shutting down as host halos grow, although its free-fall mass budgets are upper limits because gas reaching halo centers need not all accrete onto the SMBH.

Key figures to inspect

• Figure 2. This is the paper's central abundance test: it compares the redshift-dependent density of halos meeting a required inflow rate under free-fall and Bondi-Hoyle prescriptions, directly against quasar, LRD, and DCBH constraints. It makes the key claim visually clear that super-Eddington LRD interpretations require near-free-fall supply and that the low-redshift LRD decline can follow the end of cold-mode inflow.
• Figure 3. This cumulative-growth comparison shows that free-fall inflow can assemble the black-hole masses inferred for JWST AGN, LRDs, and luminous quasars along constant-number-density halo tracks. It also highlights the changing cold-mode boundary, including the tension for the most massive quasars as their hosts become too massive to retain cold-mode accretion at later epochs.
• Figure 4. The accretion-built black-hole mass functions provide the population-level consequence of the model, showing that free fall can reach the most massive early AGN but can overproduce the observed active BH mass function at later times. The figure carries the paper's principal caveat: these are supply-based upper limits, since central gas delivery, SMBH accretion, and AGN duty cycle are not equivalent.
• Figure 5. This host-halo versus accumulated-black-hole-mass diagnostic gives the most direct physical comparison with clustering-based host constraints. Observed high-redshift AGN and quasar hosts fall below or within the free-fall ceiling yet substantially above the Bondi-Hoyle prediction, reinforcing the conclusion that near-free-fall growth is necessary.