Little Red Dots at an inflection point: Ubiquitous v-shaped turnover consistently occurs at the Balmer limit

Among the most puzzling early discoveries of JWST are “little red dots” (LRDs), compact red sources that host broad Balmer emission lines, and in many cases exhibit a “V-shaped” change in slope in the rest-optical. The physical properties of LRDs currently have order-of-magnitude uncertainties, because models to explain the continuum of these sources differ immensely. Here, we leverage the complete selection of red sources in the RUBIES program, supplemented with public PRISM spectra, to study the origin of this V shape. By fitting a broken power law with a flexible inflection point, we find that a large fraction of red Hα emitters at 2 < z < 6 exhibit a strong change in slope, and that all strong inflections appear associated with the Balmer limit (0.3645 μm). Using a simple model of a reddened active galactic nucleus (AGN) with an unobscured scattered-light component, we demonstrate that the observed V shape in LRDs is unlikely to occur at any specific wavelength if the entire continuum is dominated by light from a power-law AGN continuum. In contrast, models with an intrinsic feature at the Balmer limit, such as those that are dominated by an evolved stellar population, can produce the observed spectral shapes, provided that a reddened component picks up sufficiently redward of the break. While no model can comfortably explain the full LRD spectral energy distribution, the common inflection location suggests that a single component consistently dominates the rest-frame UV optical in LRDs, and that this component is associated with T ∼ 10^4 K hydrogen.