Dusty Quasars at High Redshifts

Abstract: A population of quasars at z ~ 2 is determined based on dust luminosities vLv(7.8 um) that includes unobscured, partially obscured, and obscured quasars. Quasars are classified by the ratio vLv(0.25 um)/vLv(7.8 um) = UV/IR, assumed to measure obscuration of UV luminosity by the dust which produces IR luminosity. Quasar counts at rest frame 7.8 um are determined for quasars in the Bootes field of the NOAO Deep Wide Field Survey using 24 um sources with optical redshifts from the AGN and Galaxy Evolution Survey (AGES) or infrared redshifts from the Spitzer Infrared Spectrograph. Spectral energy distributions are extended to far infrared wavelengths using observations from the Herschel Space Observatory Spectral and Photometric Imaging Receiver (SPIRE), and new SPIRE photometry is presented for 77 high redshift quasars from the Sloan Digital Sky Survey. It is found that unobscured and obscured quasars have similar space densities at rest frame 7.8 um, but the ratio Lv(100 um)/Lv(7.8 um) is about three times higher for obscured quasars compared to unobscured, so that far infrared or submm discoveries are dominated by obscured quasars. Quasar source counts for these samples are determined for comparison to the number of submm sources that have been discovered with the SCUBA-2 camera at z ~ 2 using the Lv(100 um)/Lv(7.8 um) results together with the Bootes 7.8 um counts, and we find that only ~ 5% of high redshift submm sources are quasars, including even the most obscured quasars. Illustrative source counts are predicted to z = 10, and we show that existing SCUBA-2 850 um surveys or 2 mm surveys with the Goddard-IRAM Superconducting 2 Millimeter Observer (GISMO) survey camera should already have detected sources at z ~ 10 if quasar and starburst luminosity functions remain the same from z = 2 until z = 10.