Letter to the Editor

ALMA 360 parsec, high-frequency observations reveal warm dust in the center of a z = 6.9 quasar

¹ Departement d’Astronomie, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Max-Planck Institute for Astronomy, Königstuhl 17, 69118 Heidelberg, Germany
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁴ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3 I-20126 Milano, Italy
⁵ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903, USA
⁶ Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333 CA Leiden, The Netherlands

^⋆ Corresponding author; romain.meyer@unige.ch

Received: 3 December 2024
Accepted: 20 February 2025

Abstract

The temperature of the cold dust in z >  6 galaxies is a potential tracer of the presence of an active galactic nucleus (AGN) and stellar feedback. This is also the dominant source of uncertainty in inferring properties from the far-infrared (FIR) emission of these galaxies. We present the first resolved dust temperature map in a z >  6 quasar host galaxy. We combined new 360 parsec (pc) resolution ALMA Band 9 continuum observations with 190 pc Band 6 observations from the literature to derive the dust temperature and opacity at 0.1 <  r <  0.5 kpc scales in a z = 6.9 luminous quasar host galaxy (J2348–3054). We find that the dust temperature (and opacity) increases at the center (r <  216 pc) of the galaxy up to T_d = 73 − 88 K, potentially rising up to T_d < 149 K at r <  110 pc. The combination of the resolved and integrated FIR spectral energy distribution (SED) further reveal a dust temperature gradient and a significant contribution of the AGN hot dust torus at ν_obs ≳ 700 GHz. By taking into account the torus contribution and resolved optically thick emission, we derived the total IR luminosity (L_TIR = 8.78 ± 0.10) × 10¹² L_⊙) and corresponding star formation rate (SFR = 1307 ± 15 M_⊙yr^-1), which are at least a factor of ∼3.6 (∼0.56 dex) lower than previous measurements based on the assumption of optically thin emission. We compared the resolved dust temperature, mass, and IR luminosity profiles to simulations where they are only reproduced by models that include the AGN radiation heating the dust in the center of the galaxy. Our observations provide evidence that dust in J2348–3054 cannot be assumed to be uniformly cold and optically thin. Whether J2348–3054 is representative of the larger population of high-redshift quasars and galaxies remains to be determined with future dedicated high-resolution and high-frequency ALMA observations.