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

TL;DR

This paper presents detailed JWST observations of a high-redshift Little Red Dot, revealing dense gas and super-Eddington accretion onto a black hole, supported by multiple spectral diagnostics.

Contribution

It provides the first comprehensive spectral analysis confirming dense gas envelopes and super-Eddington accretion in a Little Red Dot at high redshift.

Findings

Detection of Thomson scattering signatures indicating high electron density

Estimation of black hole mass around 5 million solar masses

Evidence of super-Eddington accretion in an early Universe galaxy

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 ($\mu\sim2$), 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 $\lambda_{\rm Edd}\sim1.8$. Additional diagnostics support the same picture: a pronounced Balmer break ($f_{\nu,4050}/f_{\nu,3670}=2.0\pm0.1$), enhanced HeI $\lambda7065$ and $\lambda10830$ with P-Cygni absorption, Bowen-fluorescent OI $\lambda8446$-$\lambda11290$ emission requiring Ly$\beta$ 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.