Millimetre and submillimetre continuum observations of the core and hotspots of Cygnus A

We present millimetre photometry and submillimetre imaging of the central core and two hotspots in the radio lobes of the galaxy Cygnus-A. For both hotspots and the central core the synchrotron spectrum continues smoothly from the radio to a frequency of 677GHz. The spectral index of the hotspots is constant over our frequency range, with a spectral index of � � 1.0 (S� / � � ), which is steeper than at lower frequencies and represents the emission from an aged population of electrons. The core is significantly flatter, with � = 0.6 ± 0.1, suggestive of an injected spectrum with no ageing, but some evidence for steepening exists at our highest observing frequency. Although IRAS data suggest the presence of dust in Cygnus-A, our 450µm data show no evidence of cold dust, therefore the dust component must have a temperature lying between 85K and 37K, corresponding to dust masses of 1.4×10 6 M⊙ and 1.0×10 8 M⊙