We report spatially resolved dust properties of the quasar host galaxy BRI
1335-0417 at redshift z=4.4 constrained by the ALMA observations. The dust
temperature map, derived from a greybody fit to rest-frame 90 and 161 ΞΌm
continuum images, shows a steep increase towards the centre, reaching 57.1Β±0.3 K. Image decomposition analysis reveals the presence of a point source in
both dust continuum images at the same position as 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
SED better than a single component model, with dust temperatures of
87.1β18.3+34.1β K (warm component) and 52.6β11.0+10.3.β K (cold
component). The star formation rate (SFR) estimated from the cold dust
component is 1700β400+500βMββ yrβ1, a factor of three smaller
than previous estimates due to a large AGN contribution (53β15+14β%).
The unresolved warm dust component also explains the steep temperature
gradient, as the temperature profile derived after the point source subtraction
is flat. We further show that AGN-host galaxy decomposition is critical for
estimating SFR distribution, as point source subtraction reduces the estimated
central SFR surface density Ξ£SFRβ by over a factor of three.
With this correction, spatially resolved measurements of
Ξ£SFRβ and the surface gas mass density
Ξ£gasβ form a roughly linear sequence in the Kennicutt-Schmidt
diagram with a constant gas depletion time of 50-200 Myr.Comment: 25pages, 25figures, 4tables, Submitted to MNRAS, Comments are warmly
welcome