2020ApJ...900....5K

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 S₂₅₀ > 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.