Probing the Evolution of Infrared Properties of z ∼ 6 Quasars: Spitzer Observations

We present Spitzer observations of 13 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 signal-to-noise ratio in the four IRAC channels and the MIPS 24 μm band, while SDSS J0005-0006 was marginally detected in the IRAC 8.0 μm band and not detected in the MIPS 24 μm band. We find that most of these quasars have prominent emission from hot dust, as evidenced by the observed 24 μm fluxes. Their spectral energy distributions (SEDs) are similar to those of low-redshift quasars at rest frame 0.15-3.5 μm, suggesting that accretion disks and hot-dust structures for these sources have already 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 μm 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 such as 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, millimeter/submillimeter, 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.