Under the glare of a luminous quasar, the FIR continuum is still an excellent tracer of the ISM down to the central kiloparsec

ABSTRACT Contamination-free assessments of the interstellar medium and star formation in quasar host galaxies, particularly in the far-infrared, offer insights into the role of supermassive black holes in galaxy evolution. Motivated by simulations of quasar-heated dust on nuclear and galaxy-wide scales, we perform two-component (host galaxy + point source) modelling of high-resolution ($\sim$$0.1$ arcsec) Atacama Large Millimeter Array (ALMA) observations of the continuum in Band 5 ($\lambda _{\mathrm{ rest}}$$\sim 500~\mu m$) of three luminous quasars ($L_\mathrm{ {bol}}\sim 10^{47}$ erg s$^{-1}$) at $z=2$. For two quasars, we include Band 9 ($\lambda _{\mathrm{ rest}}$$\sim 154~\mu m$; 0.06 and 0.3 arcsec) data, which further constrains the unresolved nuclear component. To break the degeneracy between quasar and stellar heating of dust on extended scales, we use CO (J = 5–4) to gauge the expected contribution of star formation to the infrared luminosity. We find very good agreement, between the strength and spatial distribution of the extended continuum and its prediction from CO (J = 5–4). This is supported by our three quasars following along the $L_{\mathrm{ CO (5-4)}}-L_{\mathrm{ IR, SFR}}$ relation for inactive star-forming galaxies. As a consequence, there is no evidence for extended continuum emission that could be attributed to quasar-heated dust. As expected, the nuclear (i.e. torus) contribution is present and subdominant (e.g. 12 per cent in Band 9 for one quasar with a typical star-forming host) or non-existent (