From "The Cliff" to "Virgil": Mapping the Spectral Diversity of Little Red Dots with JWST/NIRSpec

Abstract

One of JWST’s most unexpected discoveries is the emergence of “Little Red Dots” (LRDs): compact sources at z ≳ 3 with blue rest-frame UV continua, red optical slopes, and broad Balmer emission lines that challenge standard models and suggest a population of early, unusual active galactic nuclei (AGNs). Using a comprehensive photometric selection and public NIRSpec/PRISM spectroscopy across six JWST deep fields, we identify a large sample of 118 LRDs with high-S/N spectra, enabling a population-wide analysis of their UV–optical continuum and emission lines. We find clear correlations between rest-frame UV–optical color ([0.3−0.9 µm]) and slopes: bluer LRDs have blue UV slopes (β<sub>ν,UV<sub> ∼ 0.3) and red optical slopes, while redder LRDs exhibit redder UV slopes (β<sub>ν,UV<sub> ∼ 1.1). The continuum shape shows a similar trend: redder LRDs display prominent Balmer breaks and curvature, while bluer LRDs follow power-law–like optical SEDs. From literature compilations, ∼60% of known broad-line AGNs satisfy our LRD criteria, and up to 90% of LRDs show broad Balmer lines. Emissionline diagnostics reveal a shift from high Hα/Hβ and low [Oiii]λ5007/Hβ in redder LRDs to the opposite in bluer ones, along with stronger narrow-line equivalent widths, suggesting a transition from AGN- to host-dominated emission. We fit the spectra with a two-component model combining a gasenshrouded black hole (BH) and a galaxy host. Redder LRDs require higher-luminosity, unreddened BHs and modestly reddened hosts; bluer LRDs require lower-luminosity, reddened BHs and dustfree galaxies. This framework reproduces the diversity in colors and spectral shape by varying BH luminosity, obscuration, and BH-to-host luminosity ratio.