Year: 2026
Type: preprint
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 $\mathrm{Hβ}$ and the Fe II strength ($R_{\rm FeII}$). 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-$z$ $(z \lesssim 1)$ 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 $\tt{CLOUDY}$ 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 $\mathrm{R_{FeII}}$ parameter. With this, we define a UV MS diagram using the FWHM of $\mathrm{Al III\,\lambda1860}$ and the equivalent width of $\mathrm{Fe III}$ + $\mathrm{Fe II}$. $\tt{CLOUDY}$ simulations support this result, predicting stronger Fe III emission at high $\mathrm{R_{FeII}}$. Additionally, we found object-to-object variations in the $\mathrm{Fe III}$ multiplets. Our results indicate that UV Fe III emission and $\mathrm{Al III}\,λ1860$ can serve as proxies for optical Fe II and $\mathrm{Hβ}$, 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.
Source: arXiv (Cornell University)
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Science Foundation, Ministerio de Ciencia, Innovación y Universidades, Agenția Națională pentru Cercetare și Dezvoltare, Universidad de Concepción, Peking University, Agencia Nacional de Investigación y Desarrollo, European Regional Development Fund, Agencia Estatal de Investigación
Awards: G1105844351, INSTITUTO MILENIO DE ASTROFISICA, G1622782223, G2160169353, NUCLEO MILENIO TECNOLOGIA Y INVESTIGACION TRANSVERSAL PARA ESTUDIAR AGUJEROS NEGROS, G4008337681, G410571670, G4212894969, G451917667, G491712853, G529374151, G5404395142, G5539308778, G5635532810, G5716327906, G5967599077, G624877701, G7805626310, G7863371703, G8497050386, G929308097
OpenAlex ID: W7223923409