Herschel survey of the Palomar-Green QSOs at low redshift

We investigate the global cold dust properties of 85 nearby ($z⩽0.5$) QSOs, chosen from the Palomar-Green sample of optically luminous quasars. We determine their infrared spectral energy distributions and estimate their rest-frame luminosities by combining Herschel data from 70 to 500 μm with near-infrared and mid-infrared measurements from the 2MASS and the Wide-Field Infrared Survey Explorer. In most sources the far-infrared (FIR) emission can be attributed to thermally heated dust. Single temperature modified blackbody fits to the FIR photometry give an average dust temperature for the sample of 33 K, with a standard deviation of 8 K, and an average dust mass of $7\times {10}^6{M}_{\odot }$ with a standard deviation of $9\times {10}^6{M}_{\odot }$. Estimates of star formation rates that are based on the FIR continuum emission correlate with those based on the 11.3 μm polycyclic aromatic hydrocarbon (PAH) feature, however, the star formation rates estimated from the FIR continuum are higher than those estimated from the 11.3 μm PAH emission. We attribute this result to a variety of factors including the possible destruction of the PAHs and that, in some sources, a fraction of the FIR originates from dust heated by the active galactic nucleus and by old stars.