The Convergence of Heavy and Light Seeds to Overmassive Black Holes at Cosmic Dawn

Haojie Hu, Kohei Inayoshi, Zoltan Haiman, Luis C. Ho, Ken Ohsuga

First listed 2025-03-05 | Last updated 2025-03-05

Abstract

The James Webb Space Telescope (JWST) has revealed low-luminosity active galactic nuclei (AGNs) at redshifts of $z\gtrsim 4-7$, many of which host accreting massive black holes (BHs) with BH-to-galaxy mass ($M_{\rm BH}/M_{\star}$) ratios exceeding the local values by more than an order of magnitude. The origin of these overmassive BHs remains unclear but requires potential contributions from heavy seeds and/or episodes of super-Eddington accretion. We present a growth model coupled with dark matter halo assembly to explore the evolution of the $M_{\rm BH}/M_{\star}$ ratio under different seeding and feedback scenarios. Given the gas inflow rates in protogalaxies, BHs grow episodically at moderate super-Eddington rates and the mass ratio increases early on, despite significant mass loss through feedback. Regardless of seeding mechanisms, the mass ratio converges to a universal value $\sim 0.1-0.3$, set by the balance between gas feeding and star formation efficiency in the nucleus. This behavior defines an attractor in the $M_{\rm BH}-M_{\star}$ diagram, where overmassive BHs grow more slowly than their hosts, while undermassive seeds experience rapid growth before aligning with the attractor. We derive an analytical expression for the universal mass ratio, linking it to feedback strength and halo growth. The convergence of evolutionary tracks erases seeding information from the mass ratio by $z\sim 4-6$. Detecting BHs with $\sim 10^{5-6}~M_\odot$ at higher redshifts that deviate from convergence trend would provide key diagnostics of their birth conditions.

Short digest

A semi-analytic growth model tied to dark-matter halo assembly follows M_BH/M_star for both heavy and light seeds under feedback-limited, episodic super-Eddington accretion. Despite mass loss in winds, the ratio first increases and then converges to a universal ~0.1–0.3 set by the balance between nuclear gas feeding and star-formation efficiency, forming an attractor in the M_BH–M_star plane. On this attractor, overmassive BHs grow more slowly than their hosts while undermassive seeds catch up rapidly, erasing seed-memory by z~4–6. The model predicts that 10^5–10^6 Msun BHs at higher redshift that deviate from this trend would reveal their birth conditions.

Key figures to inspect

• Figure 1 (left): Compare light vs heavy seed tracks across different halo merger histories against pre-JWST and JWST SMBH compilations to see how the model envelopes bracket observed BH masses and how halo growth rate drives early overmassiveness.
• Figure 1 (right): Inspect the evolution of M_BH/M_star for both seed classes relative to local benchmarks and the stated maximum line to see the approach toward the universal ~0.1–0.3 ratio and when tracks diverge/converge with redshift.
• Figure 2: Follow the vector-field flow in the M_BH–M_star plane at four epochs and the marked transition from super- to sub-Eddington accretion; this visualizes the attractor direction and where heavy vs light seeds accelerate or decelerate.
• Figure 3: Contrast SP1–SP3 feedback prescriptions to see how stronger/weaker outflows reshape the flow field and shift the convergence path and timescale toward the universal ratio.
• Figure 4: Use the probabilistic track contours for light/heavy seeds (with parameter spreads) and the overplotted JWST SMBHs to assess how rapidly systems approach the attractor across mass scales at successive redshifts.