1.25-mm continuum observations of very high-redshift QSOs: Is there dust at z = 4.69?

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 38. The observed flux of 10.5 ±1.5 mJy (λrest = 220 μm) 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 108 M⊙ (ω0 = 1, H0 = 50 km s-l Mpc-1). If the FIR spectrum is similar to that of F10214 + 4724, the FIR luminosity is ~ 1014 L⊙. We discuss the detect-ability of thermal dust-like spectra and show that, for a dust spectrum with Td = 80 K, the detectability 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.