The Accretion History of AGN: A Newly Defined Population of Cold Quasars

Abstract

Abstract Quasars are the most luminous of active galactic nuclei, and are perhaps responsible for quenching star formation in their hosts. The Stripe 82X catalog covers 31.3 deg2 of the Stripe 82 field, of which the 15.6 deg2 covered with XMM-Newton is also covered by Herschel/SPIRE. We have 2500 X-ray detected sources with multiwavelength counterparts, and 30% of these are unobscured quasars, with L X > 1044 erg s−1 and M B < −23. We define a new population of quasars that are unobscured, have X-ray luminosities in excess of 1044 erg s−1, have broad emission lines, and yet are also bright in the far-infrared, with a 250 μm flux density of S 250 > 30 mJy. We refer to these Herschel-detected, unobscured quasars as “cold quasars.” A mere 4% (21) of the X-ray- and optically selected unobscured quasars in Stripe 82X are detected at 250 μm. These cold quasars lie at z ∼ 1–3, have L IR > 1012 L ⊙, and have star formation rates (SFRs) of ∼200–1400 M ⊙ yr−1. Cold quasars are bluer in the mid-IR than the full quasar population, and 72% of our cold quasars have WISE W3 < 11.5 [Vega], while only 19% of the full quasar sample meets this criteria. Crucially, cold quasars have on average ∼nine times as much star formation as the main sequence of star-forming galaxies at similar redshifts. Although dust-rich, unobscured quasars have occasionally been noted in the literature before, we argue that they should be considered as a separate class of quasars due to their high SFRs. This phase is likely short-lived, as the central engine and immense star formation consume the gas reservoir. Cold quasars are type-1 blue quasars that reside in starburst galaxies.