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 α ≈ −1.0 (Sν ∝ ν α), which is steeper than at lower frequencies and represents the emission from an aged population of electrons. The core is significantly flatter, with α = −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 1.4×106M⊙ and 1.0×10 8M⊙ respectively.