Gas accretion versus black-hole-merger-driven growth modes of supermassive black holes and implications for the little red dots

¹ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Trieste (OATs), Via G.B. Tiepolo 11, I-34143 Trieste, Italy
² Centro de Ciências Naturais e Humanas – Universidade Federal do ABC, Av. dos Estados 5001 Santo André – SP, 09210-580, Brazil
³ Núcleo de Astrofísica – Universidade Cidade de São Paulo, Rua Galvão Bueno 868 São Paulo – SP, 01506-000, Brazil
⁴ Instituto de Astronomia, Geofísica e Ciências Atmosféricas – Universidade de São Paulo (IAG-USP), Rua do Matão 1226 São Paulo, 05508-090, Brazil

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 November 2025
Accepted: 16 February 2026

Abstract

We investigated the growth of central supermassive black holes (SMBHs) in galaxies, with the aim of distinguishing between gas accretion and black-hole (BH) merger-driven growth modes. We performed cosmological hydrodynamical simulations of (50 Mpc)³ comoving volumes, and analyzed how the BH feedback parameters affect the coevolution between SMBHs and their host galaxies. Starting as 10⁵ M_⊙ seeds, we find that the BHs initially grow via BH mergers to ∼10⁷ M_⊙. Gas accretion onto the BHs is low soon after seeding, and then the accretion rate increases with time and reaches the Eddington rate after 7 − 9 Gyr. The BHs then undergo very fast growth via efficient gas accretion over a time period of 600 − 700 Myr; during this period the BH mass increases 10² − 10³ times, causing their predominant growth from 10⁷ M_⊙ to (10⁹ − 10¹⁰) M_⊙. Taking into account the cosmological gas inflows and outflows, SMBHs do not grow to more than 10¹⁰ M_⊙ by z = 0, which is because of gas depletion from galaxy centers driven by BH feedback. In terms of the coevolution of SMBHs and their host galaxies along the M_BH − M_★ relation, we find that they initially lie below the relation at earlier epochs and thereby move upward toward the relation. We made some physical implications about the growth of high-z little red dots recently observed by the JWST: the normal-mass SMBHs had predominantly undergone BH-merger-driven evolution, whereas the overmassive BHs underwent periods of Eddington-limited or super-Eddington bursts of gas accretion.