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 AGN embedded within dense gas envelopes, potentially powered by super-Eddington accretion. We present GLIMPSE-17775, a luminous ($L_{\mathrm{bol}}\sim 10^{45}$ erg s${}^{-1}$) LRD at $z=3.501$ behind Abell S1063 ($\mu \sim 2$), observed with deep JWST/NIRCam and a $\sim$20 hr (80 hr de-lensed) NIRSpec/G395M spectrum. The data reveal 40+ emission and absorption features, including a rich forest of low-ionization FeII 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, the signature of Thomson scattering requiring $n_e\gtrsim 10^8$ cm${}^{-3}$. Adopting this width yields $M_{\mathrm{BH}}\sim 10^{6.7}{M}_{\odot }$, a factor of ten lower than Gaussian fits, and ${\lambda }_{\mathrm{Edd}}\sim 1.8$. Additional diagnostics support the same picture: a pronounced Balmer break ($f_{\nu ,4050}/f_{\nu ,3670}=2.0\pm 0.1$), enhanced HeI $\lambda 7065$ and $\lambda 10830$ with P-Cygni absorption, Bowen-fluorescent OI $\lambda 8446$-$\lambda 11290$ emission requiring Ly$\beta$ pumping, and 16 FeII lines matching fluorescence models. These features indicate a dense ($n\sim 10^8$ 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.

22 pages, 9 figures, 5 tables, Submitted to ApJ. NIRSpec/G395M spectrum featured in this work (from DDT #9223 PIs: S. Fujimoto & R. Naidu) is available to download from https://zenodo.org/records/17575391