Spitzer observations of high-redshift QSOs

We have observed 13 z >= 4.5 QSOs using the Multiband Imaging Photometer for Spitzer, nine of which were also observed with the Infrared Array Camera. The observations probe rest wavelengths similar to 0.6 - 4.3 mu m, bracketing the local minimum in QSO spectral energy distributions ( SEDs) between strong optical emission associated directly with accretion processes and thermal emission from hot dust heated by the central engine. The new Spitzer photometry combined with existing measurements at other wavelengths shows that the SEDs of high-redshift QSOs (z >= 4.5) do not differ significantly from typical QSOs of similar luminosity at lower redshifts z = 4.5 (z less than or similar to 2). This behavior supports other indications that all the emission components and physical structures that characterize QSO activity can be established by z = 6.4 The similarity also suggests that some QSOs at high redshift will be very difficult to identify because they are viewed along dust-obscured sight lines.