Irony at z=6.68: a bright AGN with forbidden Fe emission and multi-component Balmer absorption

TL;DR

This study presents the deepest JWST/NIRSpec spectrum of a z=6.68 AGN, revealing complex gas kinematics, broad and narrow emission lines, and metal absorption features, indicating a dense, stratified environment around a possibly overmassive black hole.

Contribution

First detailed medium-resolution JWST spectrum of a high-redshift AGN showing complex emission and absorption features, and providing insights into its gas kinematics and black hole mass estimates.

Findings

Detection of broad Balmer emission and absorption lines with complex kinematics.

Evidence for an overmassive black hole relative to dynamical mass.

Presence of a stratified narrow-line region with varying densities.

Abstract

We present the deepest medium-resolution JWST/NIRSpec spectroscopy to date of a bright Little Red Dot (LRD) AGN, Irony at z=6.68. The data reveal broad Balmer emission from H$\alpha$-H$\delta$ and Balmer absorption in H$\alpha$-H$\epsilon$. The absorption lines are kinematically split: H$\alpha$ is blueshifted while higher-order lines are redshifted suggesting complex gas kinematics; their relative ratios are inconsistent with a single, passive absorbing screen. The line depths require absorption of both the BLR and the continuum, ruling out a stellar origin, consistent with the smooth Balmer break. We fit the broad H$\gamma$-H$\alpha$ lines and find the data favor a double-Gaussian effective profile, although exponential wings are evident. Depending on the adopted profile, single-epoch virial estimates give log(M$_\bullet$/M$_\odot$)=7.86-8.39 and $\lambda_{\rm Edd}$=1.7-0.4. The dynamical mass implied by the narrow lines is low log(Mdyn/M$_\odot$)=9.1, suggesting an overmassive black hole. The narrow lines display little attenuation, A$_V<0.5$ mag; while broad H$\alpha$/H$\beta\sim9$ and the broad Balmer decrements are inconsistent with standard dust attenuation curves, suggesting collisional processes. The forbidden-line spectrum includes auroral [S II] and [N II], and a forest of [Fe II] lines. Line ratios and kinematics indicate a stratified narrow-line region with both low (n$_{\rm e}$=420 cm$^{-3}$) and high densities (n$_{\rm e}\gtrsim 6.3\times10^5$ cm$^{-3}$). We detect metal absorption lines in both the optical (Ca II and Na I) and UV range (Fe II UV1-UV3). Our results support a picture of a compact AGN embedded in a dense, high covering-factor and stratified cocoon, with complex neutral-gas kinematics. While the choice of broad-line profile affects the virial estimates of M$_\bullet$, we find the effect to be of order 0.6 dex between the different approaches.