How Little Red Dots Turn Blue: Evolution of Kelvin-Helmholtz Contracting Black Hole Envelopes and Observational Diagnostics

Abstract

James Webb Space Telescope (JWST) observations have revealed compact, red-colored, broad-line emitters known as little red dots (LRDs), while a bluer counterpart population has been referred to as little blue dots (LBDs). Their physical connection remains unclear: they may represent different evolutionary stages of the same phenomenon, distinct populations, or different phases of the broader evolution of early galaxies and their black holes (BHs). We propose a model in which a rapidly growing BH is embedded in a geometrically thick gaseous envelope accompanied by concurrent nuclear star formation. The model reproduces the observed spectral energy distributions (SEDs) of LRDs and LBDs, and naturally connects the two phases through Kelvin-Helmholtz contraction of the envelope and exposure of the nuclear accretion disk, as the gas supply declines due to external stellar activity or feedback. We predict the corresponding evolutionary tracks in the UV-optical slope ($β_{\rm UV}$-$β_{\rm opt}$) plane which bridge the apparent gap between LRDs and LBDs. These evolutionary tracks start from the LRD regime and consistently exhibit a red excursion that extends toward $(β_{\rm UV},β_{\rm opt})\sim(2,-1)$ before turning toward the LBD regime. This feature is largely insensitive to variations in the model parameters that govern the envelope structure, providing a robust signature of the transitional population. The transition lasts $\sim5$-$50\,\mathrm{Myr}$, depending primarily on the BH feedback efficiency and the gas-retention efficiency of the envelope. The abundance ratio of transitional systems to LRDs therefore provides a constraint on BH feedback and gas retention during the rapid growth of early supermassive BHs.