The Deepest GLIMPSE of a Dense Gas Cocoon Enshrouding a Little Red Dot

Abstract The detection of strong Balmer breaks and absorption features in Little Red Dots (LRDs) suggests they host active galactic nuclei embedded within dense gas envelopes, potentially powered by super-Eddington accretion. We present GLIMPSE-17775, a luminous ( L bol ∼ 10 45 erg s −1 ) LRD at z = 3.501 behind Abell S1063 ( μ ∼ 2), observed with deep JWST/NIRCam and a ∼20 hr (80 hr delensed) NIRSpec G395M spectrum. The data reveal over 40 emission and absorption features, including a rich forest of low-ionization Fe ii lines and numerous broad hydrogen recombination transitions. We use this depth to test the dense-gas interpretation through five independent diagnostics. Nearly all permitted lines show exponential wings with consistent FWHM, a signature of Thomson scattering requiring n e ≳ 10 8 cm −3 . Adopting this width yields M BH ∼ 10 6.7 M ⊙ , a factor of 10 lower than Gaussian fits, and λ Edd ∼ 1.8. Additional diagnostics support the same picture: a pronounced Balmer break ( f ν ,4050 / f ν ,3670 = 2.0 ± 0.1), enhanced He i λ 7065 and λ 10830 with P-Cygni absorption, Bowen-fluorescent O i λ 8446– λ 11290 emission requiring Ly β pumping, and 16 Fe ii lines matching fluorescence models. These features indicate a dense ( n ∼ 10 8 cm −3 ), partially ionized cocoon where scattering and fluorescence dominate line formation, providing strong evidence that at least some LRDs are powered by super-Eddington black hole growth in the early Universe.