Probing the evolution of infrared properties of z ∼ 6 quasars:: Spitzer observations

We present Spitzer observations of 13 z similar to 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 mu m band, while SDSS J0005 - 0006 was marginally detected in the IRAC 8.0 mu m band and not detected in the MIPS 24 mu m band. We find that most of these quasars have prominent emission from hot dust, as evidenced by the observed 24 mu m fluxes. Their spectral energy distributions (SEDs) are similar to those of low-redshift quasars at rest frame 0.15-3.5 mu m, 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 mu m 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.