The optical-infrared relation for active galactic nuclei: The role of contaminations

¹ Astronomical Observatory of the Jagiellonian University, Faculty of Physics, Astronomy and Applied Computer Science, ul. Orla 171, 30-244 Cracow, Poland
² National Centre for Nuclear Research, ul. Pasteura 7, 02-093 Warsaw, Poland
³ Jagiellonian University, Doctoral School of Exact and Natural Sciences, Astronomy, Cracow, Poland
⁴ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
⁵ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 31 March 2025
Accepted: 15 January 2026

Abstract

Context. Although the population of quasars evolved significantly in the past, the properties of quasars as physical objects are supposed to remain almost unchanged, which makes quasars promising candidates for cosmological tests. The X–UV luminosity relation in particular is widely used for this purpose. However, the potential of other spectral domains for this purpose remains open.

Aims. The aim of the analysis we present is to test the parameter space in order to build a well behaving OPT-IR correlation that could serve as a cosmological probe. The main objective is to calibrate the OPT-IR luminosity relation for quasars, focusing on accurate estimations of dusty torus and accretion disk luminosities. We analyzed contaminations related to host galaxies, particularly from polar dust, the interstellar medium, and stellar emission that affect the optical and infrared.

Methods. We used a sample of nearly 400 quasars with photometrical observations and spectroscopical redshift divided into four redshift bins (0.7–2.4). Full spectral energy distribution (SED) fitting was performed with the CIGALE code, and results were compared with simplified photometric luminosity estimates. The impact of non-active galactic nucleus components and the role of polar dust in the fitting process were assessed.

Results. We show that for sources with a disk luminosity above 10⁴⁵ [erg s⁻¹], the photometric estimates are consistent with SED-based values. While polar dust contributes marginally to luminosity, its presence significantly alters SED fitting, particularly the torus opening angle and cold dust properties. In the optical domain, stellar emission is the dominant contamination. In the infrared, disk emission and cold dust play major roles. We propose two empirical calibrations for the OPT–IR relation.

Conclusions. We conclude that the optical band is dominated by the accretion disk component above 10⁴⁵ or 10⁴⁶ [erg s⁻¹] depending on redshift, while IR luminosity is dominated by the dusty torus emission above 1.6 × 10⁴⁵ or 2 × 10⁴⁶ [erg s⁻¹] depending on the redshift. In this high-luminosity regime, simplified photometric methods yield reliable disk and torus luminosity estimates.