What produces the far-infrared/submm emission in the most luminous QSOs?
  The AGN. I examine the average spectral energy distributions (SEDs) of two
samples of the most powerful, unobscured QSOs at 2<z<3.5, with rest frame
optical luminosities in the 46.2<log vLv(5100Ang)<47.4 range, corresponding to
the tail of the 2<z<4 QSO optical luminosity function. I find that the AGN
could potentially account for the entire broad-band emission from the UV to the
submm, on the basis that the SEDs of these sources are similar to the intrinsic
AGN SEDs derived for lower power, lower redshift QSOs. Although this does not
preclude substantial star-formation in their host galaxies, I find that the AGN
dominates the total infrared luminosity, removing the necessity for a
star-forming component in the far-IR/submm. I argue that the origin of the
far-IR/submm emission in such powerful QSOs includes a small contribution from
the AGN torus, but is predominantly linked to dust at kpc-scales heated by the
AGN. The latter component accounts for at least 5-10 per cent of the bolometric
AGN luminosity and has an implied dust mass of the order of 10^8 solar masses.
