Little Red Dots as Direct-collapse Black Hole Nurseries

Elia Cenci, Melanie Habouzit

First listed 2025-08-20 | Last updated 2025-08-20

Abstract

The James Webb Space Telescope recently uncovered a population of massive black holes (BHs) in the first billion years after the Big Bang. Among these high-redshift BH candidates, observations have identified a class of active galactic nuclei candidates, dubbed Little Red Dots (LRDs), with extraordinarily compact gas reservoirs and peculiar spectral features. LRDs clearly emerge at redshift z<8 and their abundance declines by z<5. Recent theoretical studies have explored the link between LRDs and the formation of heavy BH seeds in the early Universe, such as direct-collapse BHs (DCBHs). Here we present results from preliminary runs for the MELIORA cosmological hydrodynamical simulations, where we implement an accurate model for DCBH formation, accounting for the Lyman-Werner radiation field and mass-inflow rates in the target host haloes. We aim to test whether or not DCBH formation could lead to systems resembling those hypothesized for LRDs. We find that the population of newly formed DCBHs in the simulations exhibits a steep decline at z<6, akin to the emergence of LRDs, primarily driven by reduced inflows. The birth of DCBHs is associated with a significant gas compaction event, followed by a phase of intense luminosity in the 200 Myr after their birth, and subsequently by the formation of the first PopIII stars in these very haloes. If these DCBHs nurseries are associated with LRDs, then it could explain their weak emission from X-rays and hot dust.

Short digest

Preliminary MELIORA cosmological simulations implement a direct-collapse BH formation model that tracks Lyman–Werner irradiation and halo inflow rates to test whether newborn heavy seeds can resemble Little Red Dots. The simulated population of newly formed DCBHs drops sharply at z<6 as inflows wane, mirroring the observed emergence-and-decline trend of LRDs. DCBH birth coincides with a strong gas-compaction event and a short, bright AGN phase lasting ~200 Myr, after which the first Pop III stars appear and the AGN fades. If LRDs trace this nursery phase, their weak X-ray and hot-dust signatures follow naturally from the scenario.

Key figures to inspect

• Figure 1 (scenario schematic): Trace the proposed timeline—gas compaction → super-massive star → DCBH → brief luminous phase → Pop III onset—to see why an LRD-like SED and weak hot-dust/X-ray output are expected only in a narrow window.
• Figure 2 (LRD vs newborn DCBH redshift trends): Compare the observed LRD redshift distribution to the simulated abundance and fraction of newborn DCBHs; the steep decline at z<6 is the key correspondence supporting the LRD–DCBH link.
• Figure 3 (halo inflow rates vs redshift): Inspect how median inflow rates fall across halo-mass bins toward lower z and correlate with the vanishing newborn-DCBH fraction; this demonstrates inflows as the primary driver of the decline.
• Figure 4 (AGN fraction vs BH age): Quantifies the short-lived luminous phase—DCBHs are most likely AGN within ~200 Myr of birth—pinning down when an object would appear LRD-like before fading as Pop III stars emerge.