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 deg${}^2$ of the Stripe 82 field, of which the 15.6 deg${}^2$ 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 $L_X>10^{44}$erg/s and $M_B<-23$. We define a new population of quasars which are unobscured, have X-ray luminosities in excess of $10^{44}$erg/s, have broad emission lines, and yet are also bright in the far-infrared, with a 250$\hat{I}¼$m 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$\hat{I}¼$m. These Cold Quasars lie at $z\sim 1-3$, have $L_{\mathrm{IR}}>10^{12}{L}_{\odot }$, and have star formation rates of $\sim 200-1400M_{\odot }$/yr. 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 $\sim 9\times$ 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 star formation rates. 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.

Accepted for publication in ApJ