20.500.12556/RUL-176489 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 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. astrophysics galaxies spectroscopy active galactic nucleus astrofizika galaksije spektroskopija aktivna galaktična jedra true false true Angleški jezik Slovenski jezik Članek v reviji 2025-12-02 12:05:17 2025-12-02 12:05:19 2025-12-21 04:15:53 0000-00-00 00:00:00 2025 0 0 12 str. no. 1 Vol. 994 Nov. 2025 0000-00-00 Zaloznikova Objavljeno NiDoloceno 0000-00-00 0000-00-00 0000-00-00 524.8 2041-8213 10.3847/2041-8213/ae13a9 259572995 Tripodi_2025_ApJL_994_L6.pdf Tripodi_2025_ApJL_994_L6.pdf 1 2CA30C0695F70CCB7C50AA7C301E0D4E 9aa7864878b4c284bc7bc8f4c6e2938938f6b0fe369aec4aebd887cd51e6338b 9bfa7913-cf70-11f0-9328-0050569b8976 https://repozitorij.uni-lj.si/Dokument.php?lang=slv&id=222848 https://iopscience.iop.org/article/10.3847/2041-8213/ae13a9 0 a74bc92a-cf70-11f0-9328-0050569b8976 https://repozitorij.uni-lj.si/Dokument.php?lang=slv&id=222849 Fakulteta za matematiko in fiziko 0 0 0