Co-evolution of Extreme Star Formation and Quasars: Hints from Herschel and the Sloan Digital Sky Survey

Abstract

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 ∼172 deg² , we have identified the far-infrared counterparts for 354 quasars, among which 134 are highly secure detections in the Herschel 250 μ {{m}} 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 cold-dust components within the host galaxies, are consistent with being heated by active star formation. In most cases (≳80%), their total infrared luminosities as inferred from only their far-infrared emissions ({L}_{IR}^({cd)}) already exceed 10¹² {L}_⊙ , 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 (≳50%-60%) have star-formation rates ≳300 {M}_⊙ yr^-1. Such extreme starbursts among optical quasars, however, is only a few percent. This fraction varies with redshift, and peaks at around z ≈ 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.