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

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 ${\lambda }_{\mathrm{Edd}}$=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$\beta \sim 9$ 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${}_{\mathrm{e}}$=420 cm${}^{-3}$) and high densities (n${}_{\mathrm{e}}\gtrsim 6.3\times 10^5$ 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.

28 pages, 15 figures, 2 tables. Submitted to ApJ