UV-to-optical insights into the black hole star model in little red dots

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Abstract
Little red dots (LRDs) are a heterogeneous class of objects, and several scenarios have been proposed 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. We conducted 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 the code Bagpipes , including blackbody (BB) emission affected by Balmer absorption, stellar and nebular emission attenuated by dust, and an active galactic nucleus (AGN) component, we assessed the performance of the black hole star (BH ∗ ) model in describing the LRD population. We adopted broad priors and therefore did 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. About 9% of the 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 a minor BB contribution. When we adopted a prior that disfavored a strong AGN continuum to enforce BH ∗ -like solutions, the percentage of BH ∗ systems increased to ∼35%, highlighting the strong degeneracy between a BH ∗ solution and alternative scenarios. Even when BH ∗ -like solutions were enforced, many LRDs still required a stellar-dominated optical continuum. This might 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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