The near-infrared spectral energy distribution of blue quasars: Determining what drives the evolution of the dusty torus

¹ Dipartimento di Fisica e Astronomia, Università di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
² INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
⁴ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
⁵ Dipartimento di Matematica e Fisica, Univeristà di Roma 3, Via della Vasca Navale, 84, 00146 Roma, Italy

^⋆ Corresponding author: bartolomeo.trefoloni@unifi.it

Received: 14 October 2024
Accepted: 5 February 2025

Abstract

A fundamental ingredient in the unified model of active galactic nuclei (AGN) is the obscuring torus, whose innermost, hottest region dominates the near infrared (NIR) emission. Characterising the change in the torus properties and its interplay with the primary AGN emission is key for our understanding of AGN physics, evolution, and classification. Its covering factor (CF) is largely responsible for the classification of AGN on the basis of the detection of broad emission lines. It is still not clear whether the torus properties evolve over time and how they relate with the accretion parameters of the nucleus. In this work, we aim at investigating the evolution of the NIR properties with the redshift (z) and the bolometric luminosity (L_bol) in a sample of AGN. To this end, we assembled a large dataset of ∼36 000 type 1 AGN between 0.5 < z < 2.9 and 45.0 < log(L_bol/(erg s)) < 48.0 with UV, optical, and near-infrared photometry. We produced average spectral energy distributions (SED) in different bins of the z − L_bol parameter space to estimate how the NIR SED evolves according to these parameters. We find that the NIR luminosity decreases for increasing L_bol at any redshift. At the same time, the shape of the NIR SED in our sample is consistent with a non-evolution with z. As a consequence, all the explored proxies for the CF exhibit significant anti-correlations with L_bol, but not with z. Additionally, the CF also shows a shallower anti-correlation with the Eddington ratio (λ_Edd), yet the current systematic uncertainties, as well as the limited dynamical range, do not allow us to precisely constrain the role of the Eddington ratio. Lastly, we derived the covering factor from the ratio between the NIR and optical luminosity and we employed it to set a lower limit for the X-ray obscuration at different redshifts.