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

TL;DR

This study presents the first high-resolution dust temperature map of a $z=6.9$ quasar host galaxy, revealing a central increase in dust temperature likely heated by the AGN, and revising the galaxy's infrared luminosity and star formation rate estimates.

Contribution

It provides the first resolved dust temperature and opacity measurements in a high-redshift quasar, demonstrating the importance of AGN heating and optical thickness in interpreting FIR data.

Findings

Dust temperature increases towards the galaxy center, reaching up to 88 K.

Total infrared luminosity and star formation rate are significantly lower than previous estimates.

Evidence suggests dust heating by the AGN dominates in the galaxy's core.

Abstract

The temperature of the cold dust in z>6 galaxies is a potential tracer of Active Galactic Nucleus (AGN) and stellar feedback, and is 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 combine new 360 pc resolution ALMA Band 9 continuum observations with literature 190 pc Band 6 observations 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, and potentially 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 $\nu_{\rm{obs}}\gtrsim 700$ GHz. By taking into account the torus contribution and resolved optically-thick emission, we derive a total infrared luminosity ($L_{TIR}=8.78\pm0.10)\times 10^{12}L_\odot$) and corresponding star-formation rate (SFR$=1307\pm15\ M_\odot\ \rm{yr}^{-1}$), that are at least a factor $\sim 3.6$ ($\sim0.56$ dex) lower than previous measurements assuming optically-thin emission. We compare the resolved dust temperature, mass and IR luminosity profiles to simulations where they are only reproduced by models in which the AGN radiation heats 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 dedicated high-resolution and high-frequency ALMA observations.