CO-EVOLUTION OF EXTREME STAR FORMATION AND QUASARS: HINTS FROM HERSCHEL AND THE SLOAN DIGITAL SKY SURVEY

Using the public data from the Herschel wide-field surveys, we study the far-infrared properties of optical-selected quasars from the Sloan Digital Sky Survey. Within the common area of similar to 172 deg(2), we have identified the farinfrared counterparts for 354 quasars, among which 134 are highly secure detections in the Herschel 250 mm band (signal-to-noise ratios >= 5). This sample is the largest far-infrared quasar sample of its kind, and spans a wide. redshift range of 0.14 <= z <= 4.7. Their far-infrared spectral energy distributions, which are due to the colddust components within the host galaxies, are consistent with being heated by active star formation. In most cases (greater than or similar to 80%), their total infrared luminosities as inferred from only their far-infrared emissions (L-IR((cd))) already exceed 10(12) L-circle dot, and thus these objects qualify as ultra-luminous infrared galaxies. There is no correlation between L-IR((cd)) and the absolute magnitudes, the black hole masses or the X-ray luminosities of the quasars, which further support that their far-infrared emissions are not due to their active galactic nuclei. A large fraction of these objects (greater than or similar to 50%-60%) have star-formation rates greater than or similar to 300 M-circle dot yr(-1). Such extreme starbursts among optical quasars, however, is only a few percent. This fraction varies with redshift, and peaks at around z approximate to 2. Among the entire sample, 136 objects have secure estimates of their cold-dust temperatures (T), and we find that there is a dramatic increasing trend of T with increasing L-IR((cd)). We interpret this trend as the envelope of the general distribution of infrared galaxies on the (T, L-IR((cd))) plane.