Ultraviolet diversity of Little Red Dots as a probe for direct-collapse black hole ages

Elia Cenci, Melanie Habouzit, Dale D. Kocevski

First listed 2026-06-02 | Last updated 2026-05-29

Abstract

Little Red Dots (LRDs) uncovered by the James Webb Space Telescope have been proposed as candidate galaxies hosting embedded accreting direct-collapse black holes (DCBHs), yet the relative ultraviolet (UV) emission of their host galaxy remains highly uncertain and diverse across the population. Using a large-scale cosmological hydrodynamical simulation from the MELIORA suite, we investigate the contribution of PopIII stars and accreting DCBHs in LRD candidates at $z>8.5$, in the rest-frame $0.2-0.6~μ\mathrm{m}$ band. We find that the UV emission from the host galaxy evolves rapidly over the first $\sim 30~\mathrm{Myr}$ following DCBH formation, reflecting the build-up of stellar mass and metal enrichment. This evolution consists of a rapid transition from initially BH-dominated systems, with negligible stellar mass, low metallicity, and high accretion rates, to progressively more developed hosts in which rapid star formation enhances the UV output and metallicity increases. After $\sim 30~\mathrm{Myr}$, the stellar continuum typically overwhelms the accreting DCBH contribution, producing bluer colours and more extended stellar distributions. As a result, UV-bright LRDs are predicted to host older DCBHs, have higher gas-phase metallicities, lower BH-to-stellar mass ratios, and lower Eddington ratios. The short-lived nature of the LRD phase places strong constraints on their emergence over cosmic time. Overall, our results suggest that DCBH ages can be constrained from the host galaxy contribution to the UV-optical spectrum of LRDs, relative to that of the accreting DCBH, and support the picture in which a DCBH evolutionary sequence is systematically encoded in emission line properties, gas-phase metallicities, and accretion states.

Short digest

Cenci, Habouzit, and Kocevski use a MELIORA cosmological hydrodynamical simulation to interpret the UV diversity of z>8.5 little red dots as an age sequence following direct-collapse black hole formation, explicitly tracking the relative contributions of PopIII stars and an accreting DCBH across rest-frame 0.2-0.6 μm. Their main result is a rapid roughly 30 Myr transition from red, BH-dominated, quasi-pristine systems with negligible stellar mass and high accretion rates to bluer hosts where fast star formation and metal enrichment boost the UV continuum. In this picture, UV-bright LRDs should preferentially host older DCBHs, higher gas-phase metallicities, lower BH-to-stellar mass ratios, and lower Eddington ratios. The payoff is that host-galaxy UV light becomes a practical clock for early heavy-seed growth, linking LRD spectral diversity to an evolutionary sequence rather than treating it as mere population scatter.

Key figures to inspect

• Figure 1 is the conceptual anchor for the paper because it lays out the proposed early-LRD to late-LRD to post-LRD sequence, showing how the UV-optical SED changes as the balance shifts from an enshrouded DCBH to newly assembled PopIII stars. It is the best overview figure for understanding the authors' central claim that UV diversity, colour evolution, and increasing stellar extent can be read as an age sequence over the first few tens of Myr after DCBH birth.
• Figure 2. Choose the first quantitative results figure that tracks the host-galaxy versus DCBH contribution in the rest-frame 0.2-0.6 μm band as a function of time after DCBH formation. This is likely where the paper moves from the schematic picture to measurable trends and should be especially useful for showing how quickly the systems leave the initially BH-dominated regime.
• Figure 4. A mid-paper diagnostic figure that connects UV brightness or colour to gas-phase metallicity and stellar build-up should be prioritized, because metallicity growth is one of the paper's main physical levers for explaining the transition from quasi-pristine newborn systems to more developed hosts. This is the figure most likely to show why bluer, more UV-luminous LRDs are interpreted as older DCBH systems rather than simply brighter ones.
• Figure 6. Prioritize the later synthesis figure that links DCBH age to BH-to-stellar mass ratio and Eddington ratio, since these are explicit bottom-line predictions in the abstract. A figure of this type would be central for LRDigest readers because it turns the qualitative age-sequence idea into concrete observables that can be compared against inferred host masses, accretion states, and future spectroscopic constraints.