Gas Accretion versus BH Merger driven Growth Modes of Supermassive Black Holes and Implications for the Little Red Dots

Paramita Barai

First listed 2026-02-17 | Last updated 2026-02-17

Abstract

We investigate the growth of central supermassive black holes in galaxies, aiming to distinguish between gas accretion versus BH merger-driven growth modes. By performing and analysing cosmological hydrodynamical simulations of $(50 ~ {\rm Mpc})^3$ comoving boxes, we also study how the BH feedback parameters affect the coevolution between SMBHs and their host galaxies. Starting as $10^5 M_{\odot}$ seeds, we find that the BHs grow initially via BH mergers to $\sim 10^7 M_{\odot}$. Gas accretion onto the BHs is initially low, then increases with time, and reaches the Eddington rate after $7-9$ Gyrs. The BHs then undergo very fast growth via efficient gas accretion over a period of $600 - 700$ Myr, when the BH mass increases $10^2 - 10^3$ times, causing their predominant growth from $10^7 M_{\odot}$ to $(10^9 - 10^{10}) M_{\odot}$. Taking into account the cosmological gas inflows and outflows, SMBHs do not grow to more than $10^{10} M_{\odot}$ by $z=0$, because of gas depletion from galaxy centers driven by AGN feedback. In terms of SMBH - host galaxy coevolution along the $M_{\rm BH} - M_{\star}$ relation, we find that they initially lie below and thereby move upward toward the relation. We make some physical implications of the growth of high-$z$ Little Red Dots recently observed by 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.

Short digest

Cosmological hydrodynamical boxes (50 Mpc)^3 are used to separate merger- from gas-accretion–driven SMBH growth and test feedback choices against SMBH–host coevolution. Seeded at 10^5 Msun, black holes first build up via BH–BH mergers to ~10^7 Msun; only after 7–9 Gyr does accretion reach Eddington, triggering 600–700 Myr bursts that amplify masses by 10^2–10^3 to 10^9–10^10 Msun. AGN-feedback-driven central gas depletion then curtails growth, keeping SMBHs below 10^10 Msun by z=0 and moving systems upward toward the M_BH–M_star relation from below. Implication for JWST little red dots: normal-mass objects are consistent with merger-dominated histories, while overmassive systems require Eddington or super-Eddington accretion episodes.

Key figures to inspect

• MBH(t) decomposed into merger versus accretion contributions: verify the early merger-dominated rise to ~10^7 Msun and pinpoint the handoff to accretion-driven growth.
• Eddington ratio or accretion rate versus cosmic time: identify the 7–9 Gyr transition to f_Edd ~ 1 and the ensuing 600–700 Myr burst responsible for 10^2–10^3 mass jumps.
• Tracks in the M_BH–M_star plane: show systems starting below the relation and migrating upward; highlight which tracks map to normal-mass versus overmassive LRD analogs.
• Central gas mass and outflow/feedback diagnostics versus time: demonstrate AGN-feedback-driven gas depletion that caps SMBH masses below 10^10 Msun by z=0.
• Parameter-sweep comparison across BH feedback efficiencies/velocities: assess robustness of the merger-then-accretion sequence and how feedback tunes final masses.