| Abstract | We report the results of 1 .25-mm observations with the IRAM 30-m telescope of five optically selected QSOs, and five radio-selected QSOs with high redshifts. Five of the QSOs have redshifts greater than 4. One of the radio-quiet, optically selected QSOs, BR1202-0725 (z=4.69), was detected at > 3σ. The observed flux of 10.5+/-1.5 mJy(λ_rest_ = 220 microns) is similar to that of the high- luminosity IRAS source F10214 + 4724 at z = 2.29 and, under the assumption that the detected emission is due to thermal emission from dust with a single temperature (80 K), the inferred dust mass is ~4 X 10^8^ M_sun_ ({OMEGA}_0_ = 1, H_0_ = 50 km s^-1^ Mpc^-1^). If the FIR spectrum is similar to that of F10214 + 4724, the FIR luminosity is ~10^14^- L_sun_. We discuss the detectability of thermal dust-like spectra and show that, for a dust spectrum with T_d_ = 80 K, the detectabillty for constant mass increases almost linearly with redshift between redshifts of 1 and 10. For the radio-loud QSOs the 1.25-mm observations indicate a steepening of the radio continuum, from a median spectral index of + 0.3 between 20 and 6 cm to ~-0.7 over the range 6 cm to 1.25 mm. |
