Discovery of Multiply Ionized Iron Emission Powered by an Active Galactic Nucleus in a z~7 Little Red Dot

Erini Lambrides, Rebecca Larson, Taylor Hutchison, Pablo Arrabal Haro, Bingjie Wang, Brian Welch, Dale D. Kocevski, Chris T. Richardson, Casey Papovich, Jonathan R. Trump, Sarah E. I. Bosman, Jane R. Rigby, Steven L. Finkelstein, Guillermo Barro, Jacqueline Antwi-Danso, Arianna Long, Anthony J. Taylor, Jenna Cann, Jeffrey McKaig, Anton M. Koekemoer, Nikko J. Cleri, Hollis B. Akins, Mic B. Bagley, Danielle A. Berg, Volker Bromm, John Chisholm, Katherine Chworowsky, Sadie Coffin, M. C. Cooper, Olivia Cooper, Isa Cox, Mark Dickinson, Henry C. Ferguson, Maximilien Franco, Jonathan P. Gardner, Norman A. Grogin, Michaela Hirschmann, Marc Huertas-Company, Intae Jung, Jeyhan S. Kartaltepe, Gourav P. Khullar, Ray A. Lucas, Elizabeth J. McGrath, Alexa M. Morales, Grace M. Olivier, Óscar A. Chávez Ortiz, Pablo G. Pérez-González, Norbert Pirzkal, Rachel S. Somerville, Brittany Vanderhoof, Benjamin J. Weiner, L. Y. Aaron Yung, Jorge A. Zavala

First listed 2025-09-11 | Last updated 2025-09-11

Abstract

Some of the most puzzling discoveries of NASA's JWST in the early Universe surround the surprising abundance of compact red sources, which show peculiar continuum shapes and broad hydrogen spectral lines. These sources, dubbed ``Little Red Dots'' or LRDs, have been the subject of intense inquiry in the literature. Any of the proposed explanations, from accreting super-massive black holes ensconced in ultra-dense gas to extremely compact star-systems, has significant implications for the earliest phases of galaxy evolution. Part of the difficulty in concretely identifying the physical mechanisms that drive their rest ultra-violet/optical spectral properties is the lack of bona fide signatures -- either star-formation or accreting super-massive black hole, that uniquely discriminate between competing interpretations. In this work, we report the discovery of several spectral features that strongly favor the existence of an accreting super-massive black hole in an LRD witnessed in the first 800 Myr of cosmic time, including several rare iron transitions and a possible [FeVII]. Additionally, we report on the properties of significant Balmer absorption and find that the small widths and relative depths of the absorption feature suggest the source of the absorber is at or beyond the outer edge of the broad-line region and does it fully cover the accreting SMBH in the center of the system. The detection of these iron features, coupled with the properties of the Balmer absorption, unveils an alternative scenario for LRDs -- one where there are direct sight-lines from the accretion disk to gas on scales at (or beyond) the broad-line gas region.

Short digest

THRILS spectroscopy of the z~7 Little Red Dot THRILS_46403 (CEERS-10444/RUBIES_49140) reveals bona fide AGN tracers: multiple Fe lines including a 4.5σ [Fe VII] and several [Fe II], plus an auroral [N II] detection at 3.1σ. Three Balmer lines show broad emission with superposed narrow absorption whose small widths and depths imply a partially covering absorber at or beyond the broad-line region. Electron-temperature diagnostics indicate O++ is markedly hotter than N+ across densities, pointing to a powerful, hard central ionizing source. Together these features establish multiply ionized iron emission from an accreting SMBH within the first 800 Myr and suggest direct sight-lines from the disk to gas at or beyond the BLR.

Key figures to inspect

• Figure 1: Inspect the 2D and 1D THRILS spectra to verify the [Fe II] suite near the Hβ–[O III] region, the isolated [Fe VII] feature, and the tentative auroral [N II]; use the multiple negative traces to assess sky/systematic residuals and line IDs.
• Figure 2: Examine the multi-component fits to Hβ, [O III], and Hα to see the separation of broad emission from the narrow absorption; note where degeneracies arise (e.g., absorption trough near line center) and how they affect inferred BLR widths and narrow-line fluxes.
• Figure 3: Compare O++ and N+ temperature curves to visualize the strong Te gradient; relate the elevated O++ temperatures to the hardness of the ionizing spectrum and to the critical density line marked for [Fe VII].
• Figure 4: Check the single-line fits establishing [Fe VII] at 4.5σ and auroral [N II] at 3.1σ; confirm centroid alignment with the systemic redshift and assess whether neighboring features or skylines could mimic these detections.