Probing the Evolution of Infrared Properties ofz~ 6 Quasars:SpitzerObservations

We present Spitzer observations of thirteen z~6 quasars using the InfraredArray Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS). Allthe quasars except SDSS J000552.34-000655.8 (SDSS J0005-0006) were detectedwith high S/N in the four IRAC channels and the MIPS 24um band, while SDSSJ0005-0006 was marginally detected in the IRAC 8.0um band, and not detected inthe MIPS 24um band. We find that most of these quasars have prominent emissionfrom hot dust as evidenced by the observed 24um fluxes. Their spectral energydistributions (SEDs) are similar to those of low-redshift quasars at rest-frame0.15-3.5 um, suggesting that accretion disks and hot-dust structures for thesesources already have reached maturity. However, SDSS J0005-0006 has an unusualSED that lies significantly below low-redshift SED templates at rest-frame 1and 3.5 um, and thus shows a strong near-IR (NIR) deficit and no hot-dustemission. Type I quasars with extremely small NIR-to-optical flux ratios likeSDSS J0005-0006 are not found in low-redshift quasar samples, indicating thatSDSS J0005-0006 has different dust properties at high redshift. We combine theSpitzer observations with X-ray, UV/optical, mm/submm and radio observations todetermine bolometric luminosities for all the quasars. We find that the fourquasars with central black-hole mass measurements have Eddington ratios oforder unity.Comment: 22 pages, 5 figures, accepted by A