Detection of dust in the most distant known radio galaxy

A search for millimetric continuum emission from eight optically selected, radio-quiet quasars and a radio galaxy with $$3.7\lt z\lt4.3$$ has been undertaken using a highly sensitive seven-channel bolometer on the IRAM 30-m Millimetre Radio Telescope. Detections of a potentially dust-rich quasar, and of 8C 1435+635, the most distant known radio galaxy, are reported. An extrapolation of the steepening centimetric radio spectrum of 8C 1435+635 accounts for less than 1 per cent of the observed 1.25-mm flux density, indicating that the emission is most likely from dust, although the present data cannot discriminate against synchrotron emission. If the emission is thermal, then the derived dust mass lies in the range, $$2\times10^9\gt M_\text d\gt8\times10^7\enspace\text M_\odot\enspace \text{for} \enspace20\lt T_\text d\lt100 \enspace\text K,\enspace \text{or}\enspace M_\text d\sim1.6\times10^8\enspace \text M_\odot\enspace \text{for}\enspace T_\text d=60 \enspace\text K$$, similar to that derived for 4C41.17, suggesting a molecular gas mass of between 4×10¹⁰ and 9×10¹¹M_ʘ 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 1.5×10⁸M_ʘ of dust (again for T_d = 60 K). Upper limits have been obtained for four quasars, corresponding to dust mass limits of about 3σ<2×10⁸M_ʘ; less useful limits have been set for a further three quasars.