Detection of dust in the most distant known radiogalaxy

Abstract

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, 2E9 < M(dust) < 8E7 Solar for 20 < T(dust) < 100K, or M(dust) ~ 1.6E8 Solar for T(dust) = 60K, similar to that derived for 4C41.17, suggesting a molecular gas mass of between 4E10 and 9E11 Solar. 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 ~ 1.5E8 Solar masses of dust (again for T(dust) = 60K). Upper limits have been obtained for four quasars, corresponding to dust mass limits of around 3 sigma < 2E8 Solar; less useful limits have been set for a further three quasars.