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

Tripodi, Roberta (author), Bradač, Maruša (author), Martis, Nicholas S. (author), Rihtaršič, Gregor (author), Felicioni, Giordano (author), Harshan, Anishya (author), Judež, Jon (author), Markov, Vladan (author)

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 λ1304 bump (i.e., a blend of O I λ1304 and Si II) at high redshift, a feature commonly seen in local AGNs but not yet reported in the early Universe. The O I λ1304/λ8446 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 Lyβ fluorescence as the dominant excitation mechanism of the low-ionization lines, with additional contributions from collisional excitation. High O I λ8446 equivalent width and weak O I λ7774 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 λ8446 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.

• Language: English
• Keywords: astrophysics, galaxies, spectroscopy, active galactic nucleus
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FMF - Faculty of Mathematics and Physics
• Publication status: Published
• Publication version: Version of Record
• Year: 2025
• Number of pages: 12 str.
• Numbering: Vol. 994, no. 1
• PID: 20.500.12556/RUL-176489
• UDC: 524.8
• ISSN on article: 2041-8213
• DOI: 10.3847/2041-8213/ae13a9
• COBISS.SI-ID: 259572995
• Publication date in RUL: 02.12.2025

Record is a part of a journal

• Title: The Astrophysical journal. Letters
• Shortened title: Astrophys. J. Lett.
• Publisher: American Astronomical Society
• ISSN: 2041-8213
• COBISS.SI-ID: 367233

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: astrofizika, galaksije, spektroskopija, aktivna galaktična jedra