SPECTRAL ENERGY DISTRIBUTIONS OF QSOs AT z > 5: COMMON ACTIVE GALACTIC NUCLEUS-HEATED DUST AND OCCASIONALLY STRONG STAR-FORMATION

We present spectral energy distributions (SEDs) of 69 QSOs at z > 5, covering a rest frame wavelength range of 0.1 mu m to similar to 80 mu m, and centered on new Spitzer and Herschel observations. The detection rate of the QSOs with Spitzer is very high (97% at lambda(rest) less than or similar to 4 mu m), but drops toward the Herschel bands with 30% detected in PACS (rest framemid-infrared) and 15% additionally in the SPIRE (rest frame far-infrared; FIR). We perform multi-component SED fits for Herschel-detected objects and confirm that to match the observed SEDs, a clumpy torus model needs to be complemented by a hot (similar to 1300 K) component and, in cases with prominent FIR emission, also by a cold (similar to 50 K) component. In the FIR-detected cases the luminosity of the cold component is of the order of 10(13) L-circle dot which is likely heated by star formation. From the SED fits we also determine that the active galactic nucleus (AGN) dust-to-accretion disk luminosity ratio declines with UV/optical luminosity. Emission from hot (similar to 1300 K) dust is common in our sample, showing that nuclear dust is ubiquitous in luminous QSOs out to redshift 6. However, about 15% of the objects appear under-luminous in the near infrared compared to their optical emission and seem to be deficient in (but not devoid of) hot dust. Within our full sample, the QSOs detected with Herschel are found at the high luminosity end in L-UV/opt and L-NIR and show low equivalent widths (EWs) in H alpha and in Ly alpha. In the distribution of H alpha EWs, as determined from the Spitzer photometry, the high-redshift QSOs show little difference to low-redshift AGN.