Little Red Dots as the Very First Activity of Black Hole Growth

Kohei Inayoshi

First listed 2025-03-07 | Last updated 2025-07-16

Abstract

The James Webb Space Telescope has detected massive black holes (BHs) with masses of $\sim 10^{6-8}~M_\odot$ within the first billion years of the universe. One of the remarkable findings is the identification of "Little Red Dots" (LRDs), a unique class of active galactic nuclei (AGNs) with distinct characteristics representing a key phase in the formation and growth of early BHs. Here, we analyze the occurrence rate of LRDs, which emerge around redshifts $z \sim 6-8$ and sharply decline at $z < 4$. We find that this trend follows a log-normal distribution, commonly observed in phenomena driven by stochastic and random factors. We propose a hypothesis that the first one or two AGN events associated with newly-formed seed BHs are observed as LRDs and their unique features fade in the subsequent episodes. This naturally explains the cosmic evolution of AGN abundance over $0 < z < 5$, which follows $\propto (1+z)^{-5/2}$ due to the cumulative effect of recurring AGN activity. The unique characteristics of LRDs are likely linked to the dense gas environments around the seed BHs, which create strong absorption features in the broad-line emission and enable super-Eddington accretion bursts, ultimately yielding the observed overmassive nature of BHs compared to the local relationship. An analytical expression for the redshift evolution of LRD abundance is provided for direct comparison with future observational constraints.

Short digest

Using 341 Little Red Dots (39 with spectroscopic redshifts) compiled from JWST surveys, this paper models their occurrence across cosmic time. The counts peak at z ~ 6–8 and fall steeply by z < 4, well described by a log‑normal distribution interpreted as the first one or two accretion episodes of newborn seed black holes. Dense circumnuclear gas naturally explains broad Balmer-line absorption, X‑ray weakness, and brief super‑Eddington bursts that yield overmassive BHs relative to local relations. Summing recurrent episodes reproduces the global AGN abundance evolution ∝ (1+z)^(−5/2) and provides an analytic form for the LRD redshift dependence for future tests.

Key figures to inspect

• Figure 1 (left): Inspect the redshift/time histogram and magenta log‑normal fit to locate the LRD peak near z ~ 6–8 and the rapid drop by z < 4; note the shaded incompleteness where the turnover enters F356W.
• Figure 2 (left): Episode‑by‑episode AGN number densities (1st–4th) show LRDs tied to the earliest episodes; verify that the summed black curve matches observed AGN abundances (Ueda 2014) while matching the Kokorev/Kocevski LRD counts.
• Figure 2 (right): Fraction of non‑LRDs versus redshift; compare the model transition after the first–second episodes with the Taylor (2024) constraint to see when systems cease to look like LRDs.
• Figure 3: UV absolute‑magnitude distributions and occurrence rates for subsets; use the vertical threshold to gauge where flux limits start biasing the high‑z tail and confirm the log‑normal trend persists for brighter cuts.
• Figure 4: Compare the preferred log‑normal occurrence model against merger‑driven decay curves to see why mergers alone cannot reproduce the steep decline.