Spatially resolved dust properties and quasar-galaxy decomposition of a hyper-luminous infrared galaxy at z=4.4

We report spatially resolved dust properties of the quasar host galaxy BRI1335-0417 at redshift z = 4.4 constrained by the Atacama Large Millimetre/submillimetre Array observations. The dust temperature map, derived from a greybody fit to rest frame 90 and 161 mu m continuum images, shows a steep increase towards the centre, reaching 57.1 +/- 0.3 K and a flat median profile at the outer regions of similar to 38 K. Image decomposition analysis reveals the presence of a point source in both dust continuum images spatially coincident with the highest temperature peak and the optical quasar position, which we attribute to warm dust heated by an active galactic nucleus (AGN). We show that a model including this warm component along with cooler dust heated by star formation describes the global spectral energy distribution better than a single-component model, with dust temperatures of 87.1(-18.3)(+34.1) K (warm component) and 52.6(-11.0)(+10.3) K (cold component). The star-formation rate (SFR) estimated from the cold dust component is 1700+500(-400)(+500) M-circle dot yr(-1), a factor of three smaller than previous estimates due to a large AGN contribution (53(-15)(+14) per cent). The unresolved warm dust component also explains the steep temperature gradient, as the temperature profile derived after the point source subtraction is flat. The point source subtraction also reduces the estimated central SFR surface density Sigma(SFR) by over a factor of three. With this correction, spatially resolved measurements of Sigma(SFR) and the surface gas mass density Sigma(gas) form a roughly linear sequence in the Kennicutt-Schmidt diagram with a constant gas depletion time of 50-200 Myr. The demonstrated AGN-host galaxy decomposition reveals the importance of spatially resolved data for accurate measurements of quasar host galaxy properties, including dust temperature, SFRs, and size.