What produces the far-infrared/submm emission in the most luminous QSOs?

Abstract

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.