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

Vasily Kokorev, John Chisholm, Rohan P. Naidu, Seiji Fujimoto, Hakim Atek, Gabriel Brammer, Steven L. Finkelstein, Hollis B. Akins, Danielle A. Berg, Lukas J. Furtak, Qinyue Fei, Tiger Yu-Yang Hsiao, Ivo Labbé, Jorryt Matthee, Julian B. Muñoz, Pascal A. Oesch, Richard Pan, Pierluigi Rinaldi, Alberto Saldana-Lopez, Daniel Schaerer, Marta Volonteri, Adi Zitrin

First listed 2025-11-10 | Last updated 2025-11-17

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 ($L_{\rm bol}\sim10^{45}$ erg s$^{-1}$) LRD at $z=3.501$ behind Abell S1063 ($μ\sim2$), 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 $n_e\gtrsim10^8$ cm$^{-3}$. Adopting this width yields $M_{\rm BH}\sim10^{6.7}M_\odot$, a factor of ten lower than Gaussian fits, and $λ_{\rm Edd}\sim1.8$. Additional diagnostics support the same picture: a pronounced Balmer break ($f_{ν,4050}/f_{ν,3670}=2.0\pm0.1$), enhanced HeI $\lambda7065$ and $\lambda10830$ with P-Cygni absorption, Bowen-fluorescent OI $\lambda8446$-$\lambda11290$ emission requiring Ly$β$ pumping, and 16 FeII lines matching fluorescence models. These features indicate a dense ($n\sim10^8$ 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.

Short digest

GLIMPSE-17775 is a lensed Little Red Dot at z=3.501 (μ≈2) with a 20 hr JWST/NIRSpec G395M spectrum revealing 40+ features, including a low-ionization Fe II forest and broad H recombination lines, at Lbol≈10^45 erg s^-1. The team tests the dense-cocoon picture via five diagnostics: nearly all permitted lines show Thomson-scattered exponential wings with a common width implying ne≳10^8 cm^-3. Using this width gives MBH≈10^6.7 M⊙ and λEdd≈1.8, with independent support from a strong Balmer break (fν,4050/fν,3670=2.0±0.1), enhanced He I λ7065 and λ10830 with P-Cygni absorption, Bowen-fluorescent O I λ8446–λ11290, and 16 Fe II lines matching fluorescence models. Together these point to a dense, partially ionized cocoon where scattering and fluorescence set the line profiles, indicating super-Eddington black-hole growth in at least some LRDs.

Key figures to inspect

• Figure 1: Inspect the NIRCam/HST cutouts for the transition from extended rest-UV to a compact core at longer wavelengths, then use the 2D/1D G395M spectra to see line crowding and the systemic redshift set by [S III] λ9071; note how the permitted lines exhibit extended wings.
• Figure 2: Panel-by-panel line fits show BIC preferences for exponential over Gaussian profiles; compare the consistent wing FWHM across permitted lines and locate the Fe II “iron forest” segments that anchor the fluorescence interpretation.
• Figure 3: Photometry+spectra around the Balmer limit—verify the strong F150W–F200W break (fν,4050/fν,3670≈2) and the reddened F200W–F277W color partly boosted by H emission; contrast GLIMPSE-17775’s break with other LRDs placed at the same rest frame.
• Figure 4: Two-component Sérsic+PSF modeling in F200W—check residuals and the wavelength-dependent point-source fraction, which rises into the LW filters and supports an unresolved AGN-dominated core atop an extended host.
• Supplementary windows in Fig. 2: Zoom to He I λ7065, λ10830 for P-Cygni absorption and to O I λ8446–λ11290 to see Bowen fluorescence requiring Lyβ pumping, both consistent with a dense, partially ionized envelope.