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 |$\gt3\sigma$|⁠. The observed flux of 10.5 ± 1.5 mJy (⁠|$\lambda_\text{rest}= 220\enspace \textmu{m}$|⁠) 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 × 108|$\text{M}\odot$| (Ω0 = 1, H0 = 50kms–1 Mpc–1). If the FIR spectrum is similar to that of F10214 + 4724, the FIR luminosity is ˜ 1014|$\text{L}\odot$|⁠. We discuss the detectability 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.