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

Abstract

Quasars are the most luminous of active galactic nuclei (AGN), 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 multi-wavelength counterparts, and 30% of these are unobscured quasars, with LX > 10⁴⁴ erg/s and MB < −23. We define a new population of quasars which are unobscured, have X-ray luminosities in excess of 10⁴⁴ erg/s, 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 Herscheldetected, unobscured quasars as “Cold Quasars”. A mere 4% (23) 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 Mdust ∼ 10⁸ −10⁹ M , have LIR > 10¹² L , and have star formation rates of 200−2000 M /yr. Cold Quasars are bluer in the mid-IR than the full quasar population, and 75% 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 4−7× as much star formation as the unobscured quasar population at similar redshifts. 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.