Little Red Dot progenitors from Compact Starbursts: A Natural Path to Early AGN Formation

Matías Liempi, Muhammad A. Latif, Dominik R. G. Schleicher

First listed 2026-06-03 | Last updated 2026-06-02

Abstract

The recent discovery of Little Red Dots (LRDs) by the James Webb Space Telescope has challenged traditional models of early galaxy and black hole co-evolution. The nature of these highly compact objects remains heavily debated, with explanations divided between dust-reddened active galactic nuclei (AGN) and extremely dense stellar populations. We perform high-resolution cosmological simulations to model the formation of LRD precursors. Motivated by recent high-redshift observations and theoretical results, we specifically explore environments characterized by high star formation efficiencies (30\% and 100\%) and confined feedback. Our simulations naturally produce highly compact galaxies with stellar masses of $10^7-6 \times 10^8 $\,M$_\odot$, with most of the mass concentrated within $200-300$ pc. We find that, in these dense environments, gas inflows, gravitational torques, and stellar dynamical friction operate on highly efficient timescales. Over a 10 Myr timescale, gas inflows can accumulate $\rm \sim 10^7 M_\odot$ at the galactic center, while gravitational torques and dynamical friction can contribute an additional $10^5-10^9$\,M$_\odot$ and $10^3-10^4$\, M$_\odot$ through the inward migration of massive stars. Assuming a conservative 10\% efficiency to account for feedback, this rapid mass accumulation can lead to the formation of a $\sim 10^6$\,M$_\odot$ central black hole, naturally giving rise to an AGN in these dense systems. Therefore, stellar and AGN interpretations of LRDs may not be mutually exclusive; rather, dense stellar systems are likely precursors to AGN.

Short digest

Using high-resolution cosmological zoom-in simulations, this paper argues that compact starbursts are natural progenitors of Little Red Dots by forming stellar systems with roughly 10^7 to 6 x 10^8 solar masses packed into just 200 to 300 pc. In the high-efficiency, confined-feedback runs, the central regions drive rapid gas inflow, strong gravitational-torque transport, and short stellar dynamical-friction times, so that within about 10 Myr they can funnel enough mass inward to plausibly assemble a central black hole of order 10^6 solar masses even under a conservative 10% net efficiency. The key implication is that the stellar and AGN pictures for LRDs need not compete: the dense stellar phase can be the route that builds the AGN. The feedback-on comparison also makes clear that preserving this compact, rapidly transporting state is crucial to the argument.

Key figures to inspect

• Figure 3. This is the clearest figure for the paper's basic claim that the simulated systems are genuinely LRD-like precursors in structure. The enclosed stellar- and gas-mass profiles show how strongly the mass is concentrated into the inner few hundred parsecs across the different star-formation-efficiency and feedback setups, which is the structural foundation for every later argument about rapid central buildup.
• Figure 5. This figure directly supports the headline result that gas inflow can pile up about 10^7 solar masses in the center on short timescales. Because it compares the radial inflow behavior across all four runs, it also shows which physical assumptions actually sustain the strong accretion needed for early black-hole growth.
• Figure 6. This is the main conclusion-driving diagnostic. By placing gas and stellar dynamical friction, crossing times, relaxation times, and gravitational-torque transport on the same radius-dependent plot, it shows that multiple inward-migration channels are efficient in the compact high-SFE case and makes the less-than-10-Myr central assembly argument quantitatively legible.
• Figure 8. This figure is the best cross-model synthesis of transport efficiency beyond pure gas inflow. The transport-rate and stellar-mass-transport profiles show how strongly the compact-starburst configurations differ from the feedback-disrupted case, clarifying why the authors conclude that dense stellar systems can feed the nucleus quickly enough to seed an AGN.
• Figure 9. This is a useful late-stage physical-diagnostic figure because it links the mass buildup to the galaxy's internal kinematics. The rotation curves show that higher star-formation efficiency deepens the central potential and supports coherent inner rotation, while feedback disrupts that structure, helping explain why compactness and confined feedback are central to the proposed LRD-to-AGN pathway.