UV-to-optical insights into the BH$^*$ model in little red dots

Rosa M. Mérida; Marcin Sawicki; Chris J. Willott; Gaia Gaspar; Kartheik G. Iyer

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arXiv · 2606.12355

UV-to-optical insights into the BH$^*$ model in little red dots

Rosa M. Mérida·Marcin Sawicki·Chris J. Willott·Gaia Gaspar·Kartheik G. Iyer

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Abstract

Little Red Dots (LRDs) are a heterogeneous class of objects, with several proposed scenarios for their physical nature and evolution. While these theories have been tested on individual LRDs using limited spectral features, a systematic Bayesian analysis of the LRD population incorporating the different models across a broad wavelength range is still lacking. In this study, we conduct a consistent ultraviolet (UV)-to-optical continuum fitting analysis of 99 LRDs at 2<z<6 using JWST/NIRSpec PRISM spectroscopy. Employing a modified version of Bagpipes-including blackbody (BB) emission affected by Balmer absorption, stellar and nebular emission attenuated by dust, and an active galactic nucleus (AGN) component-we assess the performance of the black hole star (BH*) model in describing the LRD population. We adopt broad priors and therefore do not impose any specific physical scenario. Our results show that only ~4% of LRDs with statistically robust solutions (81 objects in total) are best-fit by a BH* in the optical and a host galaxy in the UV. ~9% of LRDs show BB-dominated optical continua but lack a stellar component or exhibit AGN UV leakage. Most LRDs are dominated by stellar and/or AGN emission in the optical, with minor BB contribution. When we adopt a prior that disfavors a strong AGN continuum to enforce BH*-like solutions, the percentage of BH* systems increases to ~35%, highlighting the strong degeneracy between a BH* solution and alternative scenarios. Even when BH*-like solutions are enforced, many LRDs still require a stellar-dominated optical continuum. This could indicate limitations in the BH* model, as further supported by independent evidence, such as the detection of high-ionization emission lines in some LRDs.

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Rosa M. Mérida, Marcin Sawicki, Chris J. Willott, Gaia Gaspar, Kartheik G. Iyer. 2026-06-10. UV-to-optical insights into the BH$^*$ model in little red dots. https://arxiv.org/abs/2606.12355

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