Unified modelling of broad and narrow optical-UV emission lines from massive black holes: interpreting high-redshift JWST observations
Abstract
JWST is uncovering a large population of active galactic nuclei (AGN) at high redshift, motivating the extension of models calibrated on local sources to a broader range of physical conditions. We present a consistent suite of broad- and narrow-line photoionisation models based on ionising spectra that vary with black-hole mass and Eddington ratio, spanning sub- to super-Eddington accretion. For the broad-line region (BLR), we investigate the detectability of low-mass accreting black holes. At z=6, broad Hα may be detectable down to MBH≈105.7 M⊙ for a BH accreting at the Eddington limit under favourable assumptions. We derive line-to-accretion-luminosity corrections, with hydrogen and helium recombination lines providing more robust tracers than metal lines. We also explore which changes in black-hole properties and BLR conditions can contribute to the large Hα equivalent widths, elevated Balmer decrements, and weak high-ionisation lines observed in some high-redshift sources, finding that variations in BLR structure can substantially modify the emergent spectrum. For the narrow-line region (NLR), Hα and [O III] λ5007 are among the most promising tracers of low-mass accreting black holes, probing MBH≈105.5 M⊙ at the Eddington limit. Many classical diagnostic diagrams miss low-metallicity AGN; we identify alternative UV and optical diagnostics that more robustly separate AGN from Pop. II and Pop. III stellar photoionisation over the conditions explored. Nevertheless, NLR line ratios are driven primarily by metallicity and ionisation parameter, with only a weak dependence on black-hole mass and Eddington ratio. Looking ahead, these models are intended for Bayesian interpretation of JWST spectra and connection to cosmological simulations through physically motivated priors.
Related
- Characterizing Lyman alpha emission from high-redshift galaxies
- νGundam: A unified framework for point-redshift and probability-density-function-aware spatial correlation functions
- Redshift Identifiability from Gamma-Ray Burst Prompt Emission
- Re-interpreting the GRS 1915+105 observations within the variable TADAF scenario
- New constraints on holographic dark energy with high-redshift galaxy observations from JWST
BibTeX
@misc{plat2026unified,
title = {{Unified modelling of broad and narrow optical-UV emission lines from massive black holes: interpreting high-redshift JWST observations}},
author = {Adele Plat and Michaela Hirschmann and Emma Curtis-Lake and Anna Feltre and Stephane Charlot and Steven L. Finkelstein and Lorenzo Napolitano and Fabio Pacucci and Norbert Pirzkal and Sandra Raimundo and Anthony Taylor and Alba Vidal-Garcia and L. Y. Aaron Yung},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.12062},
eprint = {2609.12062},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.12062},
}
RIS
TY - UNPB
TI - Unified modelling of broad and narrow optical-UV emission lines from massive black holes: interpreting high-redshift JWST observations
AU - Adele Plat
AU - Michaela Hirschmann
AU - Emma Curtis-Lake
AU - Anna Feltre
AU - Stephane Charlot
AU - Steven L. Finkelstein
AU - Lorenzo Napolitano
AU - Fabio Pacucci
AU - Norbert Pirzkal
AU - Sandra Raimundo
AU - Anthony Taylor
AU - Alba Vidal-Garcia
AU - L. Y. Aaron Yung
PY - 2026
DA - 2026/09/10
PB - arXiv
DO - 10.48550/arxiv.2609.12062
UR - https://arxiv.org/abs/2609.12062
AB - JWST is uncovering a large population of active galactic nuclei (AGN) at high redshift, motivating the extension of models calibrated on local sources to a broader range of physical conditions. We present a consistent suite of broad- and narrow-line photoionisation models based on ionising spectra that vary with black-hole mass and Eddington ratio, spanning sub- to super-Eddington accretion. For the broad-line region (BLR), we investigate the detectability of low-mass accreting black holes. At z=6, broad Hα may be detectable down to M_BH≈10^5.7 M⊙ for a BH accreting at the Eddington limit under favourable assumptions. We derive line-to-accretion-luminosity corrections, with hydrogen and helium recombination lines providing more robust tracers than metal lines. We also explore which changes in black-hole properties and BLR conditions can contribute to the large Hα equivalent widths, elevated Balmer decrements, and weak high-ionisation lines observed in some high-redshift sources, finding that variations in BLR structure can substantially modify the emergent spectrum. For the narrow-line region (NLR), Hα and [O III] λ5007 are among the most promising tracers of low-mass accreting black holes, probing M_BH≈10^5.5 M⊙ at the Eddington limit. Many classical diagnostic diagrams miss low-metallicity AGN; we identify alternative UV and optical diagnostics that more robustly separate AGN from Pop. II and Pop. III stellar photoionisation over the conditions explored. Nevertheless, NLR line ratios are driven primarily by metallicity and ionisation parameter, with only a weak dependence on black-hole mass and Eddington ratio. Looking ahead, these models are intended for Bayesian interpretation of JWST spectra and connection to cosmological simulations through physically motivated priors.
ER -