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

Francesco D'Eugenio, Erica Nelson, Xihan Ji, Josephine Baggen, Jenny Greene, Ivo Labbé, Gabriele Pezzulli, Vanessa Brown, Roberto Maiolino, Jorryt Matthee, Elena Terlevich, Roberto Terlevich, Alberto Torralba, Stefano Carniani

First listed 2025-09-30 | Last updated 2025-09-30

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$α$-H$δ$ and Balmer absorption in H$α$-H$ε$. The absorption lines are kinematically split: H$α$ 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$γ$-H$α$ 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 $λ_{\rm Edd}$=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$α$/H$β\sim9$ 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.

Short digest

Deep medium-resolution JWST/NIRSpec spectra of the bright LRD “Irony” at z=6.68 reveal broad Hα–Hδ emission plus strong, multi-component Balmer absorption (Hα–Hε) whose kinematics flip sign (Hα blueshifted; higher orders redshifted), requiring absorption of both the BLR and continuum and yielding a smooth, non-stellar Balmer break. Broad-line decrements with Hα/Hβ ≈ 9 favor collisional processes, while a rich forbidden-line forest—including auroral [S II], [N II] and numerous [Fe II]—implies a stratified NLR spanning ne ≈ 420 cm⁻³ to ≳6.3×10⁵ cm⁻³ with little narrow-line extinction (AV < 0.5). Narrow-line kinematics give a low dynamical mass (log Mdyn/M⊙ ≈ 9.1) versus single-epoch MBH = 10^{7.86–8.39} and λEdd = 0.4–1.7, pointing to an overmassive BH embedded in a dense, high–covering-factor cocoon with complex neutral-gas flows. A key caveat is that the adopted BLR profile (double-Gaussian favored; exponential wings evident) shifts the virial MBH by ≈0.6 dex.

Key figures to inspect

• Balmer-series zoom (Hα–Hε): show emission plus split absorption components and their opposite velocity offsets (Hα blue vs higher-order red) to infer multi-layer neutral gas and high covering factor over both BLR and continuum.
• Broad-line profile fits around Hγ–Hα: compare double-Gaussian vs Lorentzian/exponential-wing models, highlighting changes in FWHM and resulting single-epoch MBH and λEdd estimates.
• Forbidden-line atlas: stack/identify [Fe II] lines alongside auroral [S II] and [N II]; use diagnostic ratios to visualize the stratified NLR with ne ≈ 420 cm⁻³ and ≳6.3×10⁵ cm⁻³ and minimal AV in the narrow lines.
• Balmer decrements plot: measured broad-line ratios (e.g., Hα/Hβ ≈ 9) versus standard dust curves to demonstrate the mismatch and support for collisional excitation/optical-depth effects.
• Narrow-line kinematics and dynamical mass: line-width–size or velocity dispersion analysis yielding log Mdyn/M⊙ ≈ 9.1; contrast with MBH to underscore the overmassive BH and compact host potential.