Infrared emission from z ∼ 6.5 quasar host galaxies: a direct estimate of dust physical properties

¹ Department of Physics and Astronomy, University of Bologna Via Gobetti 93/2 40129 Bologna, Italy
² INAF – Istituto Nazionale di Astrofisica Via Gobetti 93/3 I-40129 Bologna, Italy
³ Sorbonne University, UPMC Paris 6 and CNRS, UMR 7095, Institut d’Astrophysique de Paris 98b bd. Arago 75014 Paris, France
⁴ Department of Astronomy, University of Geneva Chemin Pegasi 51 1290 Versoix, Switzerland
⁵ Cosmic Dawn Center (DAWN)
⁶ DTU Space Elektrovej 327 2800 Kgs. Lyngby, Denmark
⁷ Department of Physics “G. Occhialini”, University of Milano-Bicocca Piazza della Scienza 3 I-20126 Milan, Italy
⁸ Leiden Observatory, Leiden University Niels Bohrweg 2 NL-2333 CA Leiden, The Netherlands
⁹ Max-Planck Institute for Astronomy Königstuhl 17 69118 Heidelberg, Germany
¹⁰ Department of Astronomy, School of Physics, Peking University Beijing 100871, PR China
¹¹ Kavli Institute for Astronomy and Astrophysics, Peking University Beijing 100871, China

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Received: 11 August 2025
Accepted: 27 November 2025

Abstract

Quasars at the dawn of cosmic time (z > 6) are fundamental probes for investigating the early coevolution of supermassive black holes and their host galaxy. Nevertheless, their infrared spectral energy distribution currently remains poorly constrained because the photometric coverage that probes the far-infrared wavelength range in which the dust modified blackbody is expected to peak (∼80 μm) is limited. We studied the high-frequency dust emission via a dedicated ALMA Band 8 (∼400 GHz) campaign targeting 11 quasar host galaxies at 6 < z < 7. Combined with archival observations in other ALMA bands, this program enables a detailed characterization of their infrared emission, which allowed us to derive dust masses (M_d), dust emissivity indexes (β), dust temperatures (T_d), infrared luminosities (L_IR), and associated star formation rates (SFRs). Our analysis confirmed that dust temperature is higher in this sample (34−65 K) than in local main-sequence galaxies, and this finding can be linked to the increased star formation efficiency we derived, as also suggested by the [CII]_158 μm deficit. Most remarkably, we note that the average value of T_d of this sample does not differ from the one that is observed in luminous, ultraluminous and hyperluminous infrared galaxies at different redshifts that show no signs of hosting a quasar. Finally, our findings suggest that the presence of a bright AGN does not significantly bias the derived infrared properties, although further high frequency observations with a high spatial resolution might reveal more subtle effects on subkiloparsec scales.