1996A&A...313..417K

We have observed 64 radio sources from the B3 catalogue at λ1.3mm using the bolometer of the IRAM 30-m telescope on Pico Veleta. A total of 13 objects was detected (∼20% detection rate). Out of 6 quasars, 4 were detected, and out of 16 galaxies, we have detected 2. Out of so-called "empty fields" (N=42), i.e. radio sources with optical counterparts fainter than 20.5th red magnitude, 7 have been detected. The radio continuum spectra between 151MHz and 230GHz are compiled for all sample sources. It is found that mm-weak sources generally exhibit simple power-law spectra over the whole frequency range, while powerful ones tend to possess flat or complex spectra. The observed mm flux densities generally fall on, or below, the extrapolated cm radio continuum spectra, which is interpreted in terms of a predominance of nonthermal radiation even at short mm-wavelengths. Excess λ1.3-mm emission is seen in only two sources, 1108+399 and 1437+397. Attributing this excess to thermal emission from dust, we estimate the mass of the involved dust component to be 2.0x10⁹M_☉ and 6.7x10⁹M_☉ for the two sources, respectively. This is comparable to what is inferred for a few other high-redshift objects, but a lot more than what is seen in nearby radio galaxies. We conclude that the probability of detecting thermal dust emission in distant radio galaxies is probably quite small.