Probing the Evolution of IR Properties of z~6 Quasars: Spitzer Observations

We present Spitzer observations of thirteen z~6 quasars using the Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS). All the quasars except SDSS J000552.34-000655.8 (SDSS J0005-0006) were detected with high S/N in the four IRAC channels and the MIPS 24um band, while SDSS J0005-0006 was marginally detected in the IRAC 8.0um band, and not detected in the MIPS 24um band. We find that most of these quasars have prominent emission from hot dust as evidenced by the observed 24um fluxes. Their spectral energy distributions (SEDs) are similar to those of low-redshift quasars at rest-frame 0.15-3.5 um, suggesting that accretion disks and hot-dust structures for these sources already have reached maturity. However, SDSS J0005-0006 has an unusual SED that lies significantly below low-redshift SED templates at rest-frame 1 and 3.5 um, and thus shows a strong near-IR (NIR) deficit and no hot-dust emission. Type I quasars with extremely small NIR-to-optical flux ratios like SDSS J0005-0006 are not found in low-redshift quasar samples, indicating that SDSS J0005-0006 has different dust properties at high redshift. We combine the Spitzer observations with X-ray, UV/optical, mm/submm and radio observations to determine bolometric luminosities for all the quasars. We find that the four quasars with central black-hole mass measurements have Eddington ratios of order unity.