2010A&A...518L...7S

We present a derivation of the star formation rate per comoving volume of quasar host galaxies, derived from stacking analyses of far-infrared to mm-wave photometry of quasars with redshifts 0<z<6 and absolute I-band magnitudes -22>I_AB>-32 We use the science demonstration observations of the first ∼16deg² from the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) in which there are 240 quasars from the Sloan Digital Sky Survey (SDSS) and a further 171 from the 2dF-SDSS LRG and QSO (2SLAQ) survey. We supplement this data with a compilation of data from IRAS, ISO, Spitzer, SCUBA and MAMBO. H-ATLAS alone statistically detects the quasars in its survey area at >5σ at 250,350 and 500µm. From the compilation as a whole we find striking evidence of downsizing in quasar host galaxy formation: low-luminosity quasars with absolute magnitudes in the range -22>I_AB>-24 have a comoving star formation rate (derived from 100µm rest-frame luminosities) peaking between redshifts of 1 and 2, while high-luminosity quasars with I_AB←26 have a maximum contribution to the star formation density at z∼3. The volume-averaged star formation rate of -22>I_AB>-24 quasars evolves as (1+z)^2.3±0.7 at z<2, but the evolution at higher luminosities is much faster reaching (1+z)^10±1 at -26>I_AB>-28. We tentatively interpret this as a combination of a declining major merger rate with time and gas consumption reducing fuel for both black hole accretion and star formation.