COLD DUST AROUND HIGH-REDSHIFT QUASARS

Observations of the continuum emission at 1.25 mm of 10 high-redshift (z > 3) and 10 intermediate-redshift (1.5 < z < 3) quasars, previously observed at 6 cm with the VLA, have been carried out with the SEST 15-m telescope in Chile and the IRAM 30-m antenna in Spain. Four detections at a level better than 3sigma and three marginal ones (2.8sigma and 2sigma) have been achieved. On the basis of these data it is possible to investigate the expected turnover of the AGN submillimetre spectrum, since in the quasar rest frames the 1.25-mm continuum corresponds to emission around 230-500 mum. As far as radio-quiet objects are concerned, the mechanism responsible for part of this emission is probably thermal and can be ascribed to dust grains. We discuss the possibility that this dust belongs to the quasar host galaxies, implying evidence for cold dust in these galaxies. The extinction caused by this dust could account for the higher spectral indices of the UV spectra of the detected objects with respect to the non-detected ones.