Non-LTE radiative transfer models of very luminous sources and their applicability to little red dots, quasi-stars, and similar objects

The spectral properties of little red dots (LRDs) differ from those of active galactic nuclei. LRDs may be the first stage of supermassive BH formation, where the central engine is hidden in a dense gas reservoir, in which case they are de facto quasi-stars. We investigated whether atmosphere models traditionally used for massive stars with strong winds can produce synthetic spectra morphologically similar to those of LRDs. We computed atmosphere models and synthetic spectra with the code CMFGEN. The models assume a thermalized radiation field at the inner boundary, parameterized by a temperature varying between 5000 and 12000 K. We adopted a typical luminosity of 10 10 łsun. The models are spherical, assume an expanding atmosphere, and are computed under non-local thermodynamical equilibrium (non-LTE) conditions and for several metallicities. The spectral energy distribution (SED) is different from a blackbody, with a blue optical spectrum. Broad hydrogen emission lines are produced, their wings being formed by electron scattering. The SED near the Balmer and Paschen limit is nearly continuous. A Balmer break is predicted for the coolest temperature models provided the wind density is reduced. The SED and Balmer decrement of most LRDs is reproduced by the models, provided they are dust-attenuated with Av∼1.9-2.7. Assuming the absorbed luminosity is reradiated in the infrared, the energy output at these wavelengths is consistent with observational constraints. The models predict , , , and lines. The lines at 8446 Å and 1.129 Fe ii O i O ii Ca ii O i are produced mostly by Lyβ fluorescence. The strength of the emission lines from metals depends on the input temperature, the metallicity, and the details of the radiative transfer models. CMFGEN atmosphere models predict a large number of spectral properties observed in many LRDs. They struggle to simultaneously produce a genuine Balmer break and strong emission lines. They do not account for the ultraviolet (UV) emission that is attributed to the host galaxy. Whether the models are more relevant to explain LRD spectra compared to alternative models is unclear, and leaves open the question of the physical conditions in LRDs.