Exploring the UV Fe III Emission along the Quasar Main Sequence
Abstract
The diversity of Type I AGNs can be organized along the Quasar Main Sequence (MS), a powerful framework primarily driven by the accretion rate and defined in the optical plane by the FWHM of Hβ and the Fe II strength (RFeII). Previous studies suggested that the UV Fe III evolves along this MS. However, a systematic analysis is still missing. We aim to characterize the UV Fe III emission across the MS, assess its potential as a tracer, and investigate how its properties depend on the conditions of the emitting gas. We analyzed the UV iron emission in 69 low-z (z ≲ 1) AGNs observed with the Hubble Space Telescope. We fitted the 1700-2200 Angstrom range and investigated the main correlations using principal component analysis. We complemented with ttCLOUDY photoionization simulations to explore the dependence of the iron emission on gas density, ionization parameter, metallicity, microturbulence, column density, and spectral energy distribution. We find a positive correlation between the strengths of the UV iron emission and the optical RFeII parameter. With this, we define a UV MS diagram using the FWHM of Al III λ1860 and the equivalent width of Fe III + Fe II. ttCLOUDY simulations support this result, predicting stronger Fe III emission at high RFeII. Additionally, we found object-to-object variations in the Fe III multiplets. Our results indicate that UV Fe III emission and Al III λ1860 can serve as proxies for optical Fe II and Hβ, extending the Quasar MS into the UV domain. The variations in the Fe III multiplets demonstrate that a single template may not represent all quasars. Thus, characterizing the Fe III emission and its dependence on physical conditions is essential for improving the modeling of UV AGN spectra.
Related
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- Characterizing NGC 6383: Study of fundamental properties,pre-main sequence stars, mass segregation, and age using Gaia DR3 and 2MASS
- Caught in the act: diffuse stellar and Lyα emission around a halo-less quasar in the assembling cluster CARLA J0800+4029 at z=2
BibTeX
@misc{collipal2026exploring,
title = {{Exploring the UV Fe III Emission along the Quasar Main Sequence}},
author = {Diego Martínez Collipal and Mary Loli Martínez-Aldama and Ashwani Pandey and Paola Marziani and Swayamtrupta Panda and Marco Cárdenas},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.33339},
eprint = {2610.33339},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.33339},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Exploring the UV Fe III Emission along the Quasar Main Sequence
AU - Diego Martínez Collipal
AU - Mary Loli Martínez-Aldama
AU - Ashwani Pandey
AU - Paola Marziani
AU - Swayamtrupta Panda
AU - Marco Cárdenas
PY - 2026
DA - 2026/10/23
PB - arXiv
DO - 10.48550/arxiv.2610.33339
UR - https://arxiv.org/abs/2610.33339
AB - The diversity of Type I AGNs can be organized along the Quasar Main Sequence (MS), a powerful framework primarily driven by the accretion rate and defined in the optical plane by the FWHM of Hβ and the Fe II strength (R_FeII). Previous studies suggested that the UV Fe III evolves along this MS. However, a systematic analysis is still missing. We aim to characterize the UV Fe III emission across the MS, assess its potential as a tracer, and investigate how its properties depend on the conditions of the emitting gas. We analyzed the UV iron emission in 69 low-z (z ≲ 1) AGNs observed with the Hubble Space Telescope. We fitted the 1700-2200 Angstrom range and investigated the main correlations using principal component analysis. We complemented with ttCLOUDY photoionization simulations to explore the dependence of the iron emission on gas density, ionization parameter, metallicity, microturbulence, column density, and spectral energy distribution. We find a positive correlation between the strengths of the UV iron emission and the optical R_FeII parameter. With this, we define a UV MS diagram using the FWHM of Al III λ1860 and the equivalent width of Fe III + Fe II. ttCLOUDY simulations support this result, predicting stronger Fe III emission at high R_FeII. Additionally, we found object-to-object variations in the Fe III multiplets. Our results indicate that UV Fe III emission and Al III λ1860 can serve as proxies for optical Fe II and Hβ, extending the Quasar MS into the UV domain. The variations in the Fe III multiplets demonstrate that a single template may not represent all quasars. Thus, characterizing the Fe III emission and its dependence on physical conditions is essential for improving the modeling of UV AGN spectra.
ER -