2003AJ....126...15P

We present 1.4 and 5 GHz observations taken with the Very Large Array, and observations at 250 GHz obtained with the Max Planck Millimeter Bolometer Array at the IRAM 30 m telescope, of 10 optically selected quasi-stellar objects (QSOs) at 5.0≤z≤6.28. Four sources are detected at 1.4 GHz. Two of the sources have rest-frame 1.4 GHz luminosity densities greater than 5.0x10²⁶ W/Hz, placing them in the regime of radio-loud QSOs. Both of these sources are also detected at 5 GHz. These results are roughly consistent with there being no evolution of the radio-loud QSO fraction out to z∼6. Three sources have been detected at 250 or 350 GHz by these, and previous, observations. The (sub-) millimeter flux densities for these three sources are much larger than their 1.4 GHz flux densities. The rapidly rising spectra into the (rest-frame) far-IR (FIR) argue that the observed millimeter emission is likely thermal emission from warm dust, although more exotic possibilities cannot be precluded. The implied IR luminosities are between 10¹² and 10¹³ L_☉. For J0301+0020, the radio continuum emission is clearly above that expected for a star-forming galaxy based on the radio-FIR correlation. In this case, it seems likely that the radio emission relates to the active galactic nucleus. For J0756+4104, the radio emission is within the range expected for a star-forming galaxy, while for J1044-0125 the radio upper limit is at least consistent with a star-forming galaxy. If the dust is heated by star formation, the implied massive star formation rates are between 200 and 1000 M_☉/yr. We do not detect radio emission from the reported X-ray jet associated with J1306+0356. The lack of radio emission implies that the magnetic field is well below typical equipartition values in powerful radio jets or that particle acceleration ceased between 10⁶ and 10⁷ yr ago or that the X-ray emission is not inverse Compton emission from a jet related to J1306+0356. The highest redshift source in our sample (J1030+0524 at z=6.28) is not detected at 1.4 or 250 GHz, but four fairly bright radio sources (S_1.4GHz>0.2 mJy) are detected in a 2' field centered on the QSO, including an edge-brightened (``FR II'') double radio source with an extent of about 1'. A similar overdensity of radio sources is seen in the field of the highest redshift QSO J1148+5251. We speculate that these overdensities of radio sources may indicate clusters along the lines of sight, in which case gravitational lensing by the cluster could magnify the QSO emission by a factor 2 or so without giving rise to arcsecond-scale distortions in the optical images of the QSOs.