Detection of dust in the most distant known radiogalaxy

A search for millimetric continuum emission from eight optically-selected, radio-quiet quasars and a radiogalaxy with 3.7 < z < 4.3, has been undertaken using a highly sensitive 7-channel bolometer on the IRAM 30-m Millimetre Radio Telescope. Detections of a potentially dust-rich quasar, and of 8C1435+635, the most distant known radiogalaxy, are reported. An extrapolation of the steepening centimetric radio spectrum of 8C1435+635 accounts for less than one per cent of the observed 1.25-mm flux density, indicating that the emission is most likely from warm dust, although the present data cannot discriminate against synchrotron emission. If the emission is thermal, then the derived dust mass lies in the range, 2× 10 < Md < 8× 10 7 M⊙ for 20 < Td < 100K, or Md ∼ 1.6× 10 8M⊙ for Td = 60K, similar to that derived for 4C41.17, suggesting a molecular gas mass of between 4× 10 and 9× 10 M⊙. The quasar, PC2047+0123 at z = 3.80, has no detectable centimetric emission and the 1.25-mm continuum detected here probably also originates from 1.5× 10 M⊙ of dust (again for Td = 60K). Upper limits have been obtained for four quasars, corresponding to dust mass limits of around 3 σ < 2×10 M⊙; less useful limits have been set for a further three quasars.