A deep dive down the broad-line region: permitted OI, CaII and FeII emission in an AGN Little Red Dot at z=5.3

Roberta Tripodi, Maruša Bradač, Francesco D'Eugenio, Nicholas Martis, Gregor Rihtaršič, Chris Willott, Laura Pentericci, Bianca Moreschini, Maxim Markevitch, Yoshihisa Asada, Antonello Calabrò, Guillaume Desprez, Giordano Felicioni, Gaia Gaspar, Anthony H. Gonzalez, Anishya Harshan, Xihan Ji, Jon Judež, Brian C. Lemaux, Alessandro Marconi, Vladan Markov, Rosa M. Merida, Lorenzo Napolitano, Gaël Noirot, Massimiliano Parente, Annika H. G. Peter, Luke Robbins, Andrew Robertson, Ghassan T. E. Sarrouh, Marcin Sawicki

First listed 2025-07-28 | Last updated 2025-11-07

Abstract

We present a spectroscopic analysis of a broad-line active galactic nucleus (AGN) selected as little red dot at $z = 5.3$ behind the Bullet cluster (Bz5.3), based on JWST/NIRCam and NIRSpec data. The detection of strong FeII, OI, and CaII 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 $\lambda1304$ bump (i.e., blend of OI$λ$1304 and SiII) at high redshift, a feature commonly seen in local AGNs but not yet reported in the early Universe. The OI$λ$1304/$\lambda8446$ 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 $A_V \sim 0.4$--$1.0$. We identify Ly$β$ fluorescence as the dominant excitation mechanism of the low-ionization lines, with additional contributions from collisional excitation. High OI$λ$8446 equivalent width and weak OI$λ$7774 support this interpretation. The detection of iron emission, whether from broad permitted or narrow forbidden lines, supports the presence of a stratified BLR, as also recently proposed in local LRDs. Photoionization modeling of OI$λ$8446 and CaII 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.

Short digest

JWST/NIRCam imaging and NIRSpec prism spectroscopy of the Bullet‑cluster LRD Bz5.3 (zspec=5.2907) reveal broad Balmer emission plus strong low‑ionization Fe II, O I, and Ca II triplet lines, confirming a bona fide BLR around an accreting SMBH. The work reports the first high‑z detection of the O I λ1304 bump (O I+Si II), alongside prominent O I λ8446 and Ca II λλ8498,8542,8662. A markedly low O I λ1304/λ8446 photon ratio (0.1–0.3) implies internal dust with AV≈0.4–1.0 and points to Lyβ fluorescence as the dominant excitation, consistent with high EW O I λ8446 and weak O I λ7774. Photoionization modeling of O I and Ca II favors multiple BLR gas phases with distinct densities/ionization states, supporting a stratified BLR at early times.

Key figures to inspect

• NIRSpec prism 2D/1D spectrum around the O I λ1304 bump: verify the blended O I+Si II ‘λ1304’ feature at z=5.2907, its breadth relative to nearby Fe II/ Balmer structure, and alignment with the systemic redshift.
• Spectral window around O I λ8446 and the Ca II triplet (λ8498, λ8542, λ8662): check detection significance, relative fluxes, and whether line widths/centroids track each other—key for co-spatial LIL gas in the BLR.
• OI excitation diagnostics: inspect the measured photon ratio O I λ1304/λ8446 and the weakness of O I λ7774 versus model expectations (fluorescence vs collisional), and how these map to the quoted AV≈0.4–1.0.
• Fe II emission decomposition: look at the Fe II pseudo‑continuum/line template fit to judge whether iron is predominantly broad permitted versus any narrow forbidden contribution, and how its kinematics compare to O I/Ca II.
• NIRCam cutouts with centroid overlays: assess compactness and any wavelength‑dependent centroid shifts relative to F444W, supporting a nuclear point source consistent with an LRD BLR.