Sensitive observations at 1.4 and 250 GHz of z > 5 QSOs

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 5.0 × 10²⁶ W Hz^-1, 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^-1. 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.