The near-infrared SED of blue quasars: what drives the evolution of the dusty torus?

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 main 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 ($L_{\rm bol}$) of the AGN. To this end, we assembled a large dataset of $\sim$36,000 Type 1 AGN between $0.5<z<2.9$ and $45.0<\log(L_{\rm bol} / (\rm erg / s))<48.0$ with UV, optical and near-infrared photometry. We produced average spectral energy distributions (SED) in different bins of the $z-L_{\rm bol}$ parameter space to estimate how the NIR SED evolves according to these parameters. We find that the NIR luminosity decreases for increasing $L_{\rm bol}$ 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 $L_{\rm bol}$, but not with $z$. Additionally, the $CF$ also shows a shallower anti-correlation with the Eddington ratio ($\lambda_{\rm Edd}$), yet 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.