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

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.

• Jezik: Angleški jezik
• Ključne besede: astrophysics, galaxies, spectroscopy, active galactic nucleus
• Vrsta gradiva: Članek v reviji
• Tipologija: 1.01 - Izvirni znanstveni članek
• Organizacija: FMF - Fakulteta za matematiko in fiziko
• Status publikacije: Objavljeno
• Različica publikacije: Objavljena publikacija
• Leto izida: 2025
• Št. strani: 12 str.
• Številčenje: Vol. 994, no. 1
• PID: 20.500.12556/RUL-176489
• UDK: 524.8
• ISSN pri članku: 2041-8213
• DOI: 10.3847/2041-8213/ae13a9
• COBISS.SI-ID: 259572995
• Datum objave v RUL: 02.12.2025

Gradivo je del revije

• Naslov: The Astrophysical journal. Letters
• Skrajšan naslov: Astrophys. J. Lett.
• Založnik: American Astronomical Society
• ISSN: 2041-8213
• COBISS.SI-ID: 367233

Licence

• Licenca: CC BY 4.0, Creative Commons Priznanje avtorstva 4.0 Mednarodna
• Povezava: http://creativecommons.org/licenses/by/4.0/deed.sl
• Opis: To je standardna licenca Creative Commons, ki daje uporabnikom največ možnosti za nadaljnjo uporabo dela, pri čemer morajo navesti avtorja.

Sekundarni jezik

• Jezik: Slovenski jezik
• Ključne besede: astrofizika, galaksije, spektroskopija, aktivna galaktična jedra