Accretion History of AGN: Understanding the Far-IR Properties of Unobscured Quasars

We aim to determine the intrinsic far-IR emission of unobscured quasars over cosmic time. Using a 16 deg^2 region of the Stripe 82 field surveyed by XMM-Newton and Herschel, we identify >1800 X-ray luminous AGN from z= 0−3.  However, only 10 percent of our AGN are detected both in the X-ray and IR, where the IR is necessary to constrain host galaxy properties such as star formation rate and gas mass. On the other hand, our understanding of the ISM in the other 90 percent of sources is limited. Because our sample is undetected in the far-IR, we stack Herschel/SPIRE maps to determine the average IR luminosity of our sources separated into bins of X-ray luminosity and redshift. We create median SEDs from the optical to the far-IR for our unobscured quasars from z= 0−3, and we measure the average star formation rate, dust mass, and AGN bolometric luminosity. We find that unobscured quasars on average have increasing SFRs with increasing bolometric luminosity, and this relationship is insensitive to redshift. Unobscured quasars surprisingly have similar gas masses as star forming galaxies at the same IR luminosity. However, unobscured quasars fall below the main sequence of star formation, indicating a lower star formation efficiency. This suggests that, on average, unobscured quasars are beginning to quench star formation,  but this quenching does not appear to be due to any dissipation of the ISM.