Little Red Dots as Globular Clusters in Formation

TL;DR

This work investigates whether Little Red Dots (LRDs), JWST-detected high-redshift compact sources, are globular clusters in formation rather than accreting supermassive black holes. The authors demonstrate that a young stellar cluster can supply the rest-frame UV while a short-lived supermassive star (SMS) can reproduce the rest-frame optical continuum via optically thick winds, aligning with LRD spectra and multiwavelength constraints. They propagate the z~5–7 UV luminosity function through a simple mass-loss-driven evolution to z~0, obtaining a present-day mass function with a high-mass exponential cutoff and a turnover near a few times ten^5 M_sun, consistent with local globular clusters; the inferred total number density is ~0.1–0.3 Mpc^-3, similar to the local GC population. The model makes testable predictions for chemical abundance patterns, including He and N enhancements and Na-O and Al-Mg anti-correlations, and discusses the potential presence of IMBH remnants; these predictions offer a pathway to connect distant LRDs with local GC physics and extreme early-Universe stellar processes.

Abstract

Little Red Dots (LRDs), among the most enigmatic high-redshift discoveries by JWST, are commonly believed to be powered by accreting supermassive black holes. Here, we explore the possibility that these sources are globular clusters in formation, with rest-frame UV arising from a very young stellar population and rest-frame optical from a short-lived supermassive ($>10^4$ M$_\odot$) star. The spectral profiles of LRDs are broadly consistent with this scenario, though the observed temperatures and bolometric luminosities favor emission reprocessed by optically thick, continuum-driven winds not fully captured by current models. The LRD $z\sim5-7$ UV luminosity function naturally evolves, under standard evolutionary and mass-loss prescriptions, into a present-day mass function with a turnover at $\log_{10}(M_\ast$/$M_\odot)=5.3$ and an exponential cutoff at high masses, consistent with local globular-cluster populations. We estimate the total present-day number density of LRDs formed across all redshifts to be $\approx0.3$ Mpc$^{-3}$, similar to local globular clusters. The observed LRD redshift range matches the age distribution of metal-poor globular clusters, without current LRD counterparts to the metal-rich population. If LRDs are globular clusters in formation, we predict chemical abundance patterns characteristic of multiple stellar populations, including enhanced He and N, and potential Na-O and Al-Mg anti-correlations. These results offer a local perspective to explore this surprisingly abundant population of distant sources, and a potential new window into extreme stellar astrophysics in the early Universe.

Figures (4)

• Figure 1: Illustration of a bright LRD A2744-45924 labbe24 as a globular cluster in formation martins. The gold line shows a young star cluster with nebular continuum self-consistently added, while the dark-blue line shows a supermassive star model. The combined shape (red line) highlights a ''V-shaped'' morphology as the star cluster dominates the UV and the supermassive star dominates the optical. The supermassive star model is not tuned to match the spectrum. Cooler supermassive star models may reduce the mismatch near 3500 Å, but no such models currently exist (see \ref{['sms']}).
• Figure 2: Comparison of the effective temperature ($T_{\rm eff}$) and the bolometric luminosity ($L_{\rm{bol}}$) as measured from modified blackbody fits to a sample of Little Red Dots degraaf. The solid light-blue line illustrates a model for the evolution of an accreting supermassive star in hydrostatic equilibrium nandal. The SMS model is significantly cooler and less luminous than the LRDs, possibly because it does not account for a dense wind (\ref{['sms']}). The blue-dashed line shows the Hayashi Line, or the maximum luminosity of a fully convective star in hydrostatic equilibrium. In dashed gray, we show lines of constant radius (500 and 2,500 au), while the dotted gray horizontal lines show the Eddington Luminosity of an object with $\log($M$_\ast$/M$_\odot) = 4, 5, 6,$ and 7. The gold-dashed line shows the Eddington Luminosity for a $1,000$ au object with mass increasing from $10^4$ to 10$^7$ M$_\odot$. The Little Red Dot population is consistent with a $10^{4-7}$ M$_\odot$ star emitting at, or above, the Eddington Luminosity.
• Figure 3: The estimated evolution of the LRD mass function from $z\sim7$ (solid gold line) to $z\sim0$ (blue curve). We use the observed LRD UV luminosity function kokorev24 and an extreme UV mass-to-light ratio that is consistent with a young globular cluster with an active supermassive star. We compare the predicted $z\sim0$ mass function to globular clusters in the Milky Way harris and in Virgo jordan. The extrapolation of the $z\sim7$ galaxy stellar mass function navarro to these M$_\ast$ has a distinct shape from the LRDs. All mass functions have been normalized by the total number to emphasize their shapes. LRDs can plausibly match the observed local globular cluster mass function using standard assumptions about their mass-loss (see \ref{['shape']}).
• Figure 4: Comparison of the formation redshifts for Little Red Dots kocevski to globular clusters in the Milky Way forbes. We conservatively split the Milky Way globular clusters as metal-rich ([Fe/H] $>-1.4$; dashed light-blue) and metal-poor ([Fe/H] $< -1.4$; dark-blue line) to emphasize the distinct populations. The formation redshift of metal-poor Milky Way globular clusters roughly matches the redshift distribution of LRDs.