Finding Rare AGN: Investigating the Evolution of Cold Quasars

The classic major merger scenario states that the most luminous quasars are formed through the coalescence of two gas-rich disk galaxies, in which a starburst phase precedes the unobscured quasar phase. Cold quasars break this picture as they are unobscured quasars yet have significant star formation. Our previous understanding of cold quasars is derived from a small sample size from the Stripe82X survey. I expand the sample using the North field from the XXL-XMM survey. I created a crossmatched catalog of 1005 X-ray luminous AGN with multiwavelength counterparts including photometry in all three Herschel SPIRE bands. Within the XXL sample, I observed 175 cold quasars, greatly expanding the original sample size from 30 observed in Stripe82X. I constructed spectral energy distributions (SED) for each source to estimate host galaxy properties including star formation rate, dust mass, stellar mass, and infrared luminosity. We also measure black hole masses from SDSS spectra. We find that: 1) most cold quasars have black hole masses that are significantly larger than would be expected from their stellar mass; 2) cold quasars predominantly lie well above the main sequence of star formation; and 3) cold quasars have elevated gas masses compared with normal unobscured quasars. Through examining the far-infrared emission and X-ray emission of cold quasars over cosmic time, I demonstrate that they do not fit into the standard picture of quasar evolution.