A Deep Dive down the Broad-line Region: Permitted O <scp>I</scp> , Ca <scp>II,</scp> and Fe <scp>II</scp> Emission in an Active Galactic Nucleus Little Red Dot at <i>z</i> = 5.3

Abstract We present a spectroscopic analysis of a broad-line active galactic nucleus (AGN) selected as a little red dot (LRD) at z = 5.3 behind the Bullet cluster (Bz5.3) based on JWST/NIRCam and NIRSpec data. The detection of strong Fe ii , O i , and Ca ii triplet emission lines, along with the evidence of broad Balmer lines, provides strong evidence of a broad-line region (BLR) and an accreting supermassive black hole. Notably, we report the first detection of the λ 1304 bump (i.e., a blend of O i λ 1304 and Si ii ) at high redshift, a feature commonly seen in local AGNs but not yet reported in the early Universe. The O i λ 1304/ λ 8446 photon ratio provides an independent measurement of dust attenuation in galaxies. In Bz5.3, this ratio is highly suppressed (0.1–0.3), implying significant internal dust extinction, with estimated dust attenuation A V ∼ 0.4–1.0. We identify Ly β fluorescence as the dominant excitation mechanism of the low-ionization lines, with additional contributions from collisional excitation. High O i λ 8446 equivalent width and weak O i λ 7774 support this interpretation. The detection of iron emission, whether from broad permitted or narrow forbidden lines, supports the presence of a stratified BLR, as also recently proposed in local LRDs. Photoionization modeling of O i λ 8446 and Ca ii further suggests the coexistence of multiple gas phases with distinct densities and ionization states, highlighting the complexity of the BLR. Bz5.3 thus offers a rare window into early AGN activity and BLR physics at early times.

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Journal
The Astrophysical Journal Letters
Published
2025-11-11
DOI
https://doi.org/10.3847/2041-8213/ae13a9
Citations
3
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
3.46
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article

A Deep Dive down the Broad-line Region: Permitted O I , Ca II, and Fe II Emission in an Active Galactic Nucleus Little Red Dot at z = 5.3

Gaël Noirot, Anishya Harshan, Francesco D’Eugenio, Maruša Bradač et al.
3 citations
The Astrophysical Journal Letters
Astrophysical Phenomena and Observations
3.46
article

A Deep Dive down the Broad-line Region: Permitted O I , Ca II, and Fe II Emission in an Active Galactic Nucleus Little Red Dot at z = 5.3

Gaël Noirot, Anishya Harshan, Francesco D’Eugenio, Maruša Bradač, Andrew Robertson, Vladan Markov, L. Pentericci, G. Desprez, Massimiliano Parente, Chris J. Willott, Lucy Robbins, Ghassan T. E. Sarrouh, Xihan Ji, Antonello Calabrò, Yoshihisa Asada, Rosa M. Mérida, A. Marconi, Roberta Tripodi, Marcin Sawicki, Gaia Gaspar, Maxim Markevitch, Nicholas S. Martis, Annika H. G. Peter, Anthony H. Gonzalez, Gregor Rihtaršič, B. Moreschini, Lorenzo Napolitano, B. C. Lemaux, Jon Judež, Giordano Felicioni
article en
3 citations

Abstract

Abstract We present a spectroscopic analysis of a broad-line active galactic nucleus (AGN) selected as a little red dot (LRD) at z = 5.3 behind the Bullet cluster (Bz5.3) based on JWST/NIRCam and NIRSpec data. The detection of strong Fe ii , O i , and Ca ii triplet emission lines, along with the evidence of broad Balmer lines, provides strong evidence of a broad-line region (BLR) and an accreting supermassive black hole. Notably, we report the first detection of the λ 1304 bump (i.e., a blend of O i λ 1304 and Si ii ) at high redshift, a feature commonly seen in local AGNs but not yet reported in the early Universe. The O i λ 1304/ λ 8446 photon ratio provides an independent measurement of dust attenuation in galaxies. In Bz5.3, this ratio is highly suppressed (0.1–0.3), implying significant internal dust extinction, with estimated dust attenuation A V ∼ 0.4–1.0. We identify Ly β fluorescence as the dominant excitation mechanism of the low-ionization lines, with additional contributions from collisional excitation. High O i λ 8446 equivalent width and weak O i λ 7774 support this interpretation. The detection of iron emission, whether from broad permitted or narrow forbidden lines, supports the presence of a stratified BLR, as also recently proposed in local LRDs. Photoionization modeling of O i λ 8446 and Ca ii further suggests the coexistence of multiple gas phases with distinct densities and ionization states, highlighting the complexity of the BLR. Bz5.3 thus offers a rare window into early AGN activity and BLR physics at early times.

The Astrophysical Journal LettersVol. 994(1)
Tufts University (US), Herzberg Institute of Astrophysics (CA), Space Telescope Science Institute (US), Goddard Space Flight Center (US), Waseda University (JP), University of Ljubljana (SI), University of Groningen (NL), York University (CA), University of Cambridge (GB), Arcetri Astrophysical Observatory (IT), Astronomical Observatory of Rome (IT), Trieste Astronomical Observatory (IT), University of Florida (US), Gemini North Observatory (US), Université Bourgogne Franche-Comté (FR), Carnegie Observatories (US), Istituto Nazionale di Fisica Nucleare, Sezione di Firenze (IT), Institute for Fundamental Physics of the Universe (IT), University of Florence (IT), The Ohio State University (US), Saint Mary's University (CA), University of California, Davis (US)
Openalex Percentile: Top 7%
Astrophysical Phenomena and Observations
3.46
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