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

TL;DR

The study tests the dense-gas cocoon model for high-redshift AGN by delivering an ultra-deep JWST/NIRSpec spectrum of GLIMPSE-17775 at z = 3.501 behind a modest lensing magnification of ∼2. The spectrum reveals 40+ emission and absorption features, including a dense Fe II forest, with line profiles that exhibit exponential wings consistent with Thomson scattering in $n_e obreak\gtrsim 10^8$ cm$^{-3}$; this yields a revised black-hole mass of $M_{ m BH} obreak\sim 10^{6.7} m\,M_\odot$ and an Eddington ratio of $ obreak\lambda_{ m Edd} obreak\sim 1.8$. Additional diagnostics — a pronounced Balmer break with $f_{ u,4050}/f_{ u,3670} obreak= obreak 2.0\,\nobreak obreak obreak obreak extpm 0.1$, strong He I with P-Cygni absorption, Bowen-fluorescent O I emission, and a rich Fe II forest — corroborate a dense, partially ionized cocoon shaping line formation. Collectively, these results provide direct spectroscopic confirmation that at least some LRDs are powered by super-Eddington black-hole growth in the early universe, embedded within optically thick envelopes surrounding the accreting engine.

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.

Figures (4)

• Figure 1: Top:JWST/NIRCam and HST 2$.\!\!^{\prime\prime}$0 stamps and the RGB short (SW) and long wavelength (LW) color images comprised of the F115W, F150W, F200W and F277W, F356W, and F444W bands, respectively. MSA shutters for both configurations covering GLIMPSE-17775 are shown in blue and red, respectively. The source morphology is resolved and extended up to $\sim2\mu$m, and then appears to transition to a more PSF-dominated and compact shape, echoing a growing sample of LRDs with extended rest-UV morphology matthee23rinaldi25ajuodzbalis24_rosettalabbe24 Thus likely hinting at a presence of the host-galaxy in the filters covering rest-UV. In each panel we show the total AB magnitude as presented in the GLIMPSE catalog kokorev25aatek25. The source is exceptionally bright (M444$\sim23.6$) and is detected in most JWST bands at $>100\sigma$. Middle: 2D MSA G395M spectra covering GLIMPSE-17775. Bottom: Combined 1D spectrum kokorev23cde_graaff24 of the LRD in the observed frame. We show the data in black, and the uncertainty with a black shaded region. Fixing the systemic redshift to the $[$S iii$]$$\lambda$9071 line - $z_{\rm spec}$ = $3.50102\pm0.00019$ - we show the positions and label the prominent emission with significant ($\geq3\sigma$) detections as solid vertical lines. Iron lines are shown separately in green. Due to the sheer number of features, not all could be labeled, we show all of them in \ref{['fig:fig_lines']}. Emission lines for which we only obtain an upper limit are shown with dashed lines.
• Figure 2: A staggering abundance of spectral lines in GLIMPSE-17775 at z=3.501. For each spectral window defined in \ref{['sec:lines']}, we show the data (black), the best-fit narrow and broad components (dark purple and blue), and the total best-fit model including the continuum (red). All broad components were fit with models allowing exponential wings. The $\Delta$BIC between exponential and Gaussian fits is reported in the top right of each panel, negative number indicates a preference for an exponential profile. The lower panels display the uncertainty-weighted residuals for each fit.
• Figure 3: The diversity of Balmer breaks in LRDs. Black points show the HST and JWST GLIMPSE photometry of GLIMPSE-17775. Blue line shows best-fit EAZY SED fit to the photometry only, fixing the redshift to the $z_{\rm spec}$. Maroon line show the combined and photometry-corrected G395M spectrum. While the red color in F200W-F277W is partially influenced by a bright H$\alpha$ line, the Balmer break between F150W and F200W is still prominent. We further show spectra of various other LRDs labbe24naidu25_bh*taylor25wang24degraaff25_cliff, all shifted to $z=3.501$ and normalized at 5100 Å. Finally, we show HST (blue) and JWST (orange) filter transmission curves below.
• Figure 4: Top: Two-component Sérsic + point-source fit to the F200W morphology of GLIMPSE-17775. Panels show, from left to right, the data, best-fit model, model with the extended component removed, and residuals. Bottom: Fractional point-source contribution versus rest-frame wavelength. Shaded regions mark SW (blue) and LW (red) detectors, where the lower resolution of LW filters hinders reliable two-component decomposition.