THE NATURE OF THE MILLIMETER EMISSION IN NGC 4102, NGC 4418, NGC 6000AND MRK 231

Millimetre and submillimetre photometry of four galaxies with bright c ompact infrared nuclei is presented. The two galaxies with starburst n uclei have 60 mum-1 mm spectra well fitted by isothermal emission from cool (approximately 35 K) dust grains with a nu2 emissivity dependenc e and no evidence of emission from a much colder dust component. Non-t hermal nuclear emission and free-free emission arising from the nuclea r H II regions become significant at wavelengths longer than 1 mm, whe re they can make a measurable contribution to the flux. The Seyfert ga laxy Mrk 231 shows clear excess emission beyond 1 mm above the dust co ntinuum fitted to the far-infrared and submillimetre photometry. This component has a flux level that is consistent with an extrapolation of the radio spectrum attributed to synchrotron emission. The data on th e heavily obscured nucleus of NGC 4418 are more ambiguous. Adequate re presentations of the far-infrared to millimetre dust emission can be o btained with a dust emissivity dependence of between nu and nu2, depen ding upon the dust opacity adopted at far-infrared wavelengths. The ph otometry shows a weak excess above the dust continuum at wavelengths b eyond 1300 mum, which can partially be accounted for by an extrapolati on of the radio spectrum; the residual excess may be due to free-free emission. By equating the sizes of the dust emission regions to those of the compact radio cores in these galaxies, fits to the dust emissio n spectra are obtained that are optically thick at mid- and far-infrar ed wavelengths in Mrk 231 and NGC 4418, but which are in approximate a greement with the extinction estimated from the 9.7-mum silicate absor ption features. While there is no evidence of emission from a cold (< 20 K) dust component in any of these galaxies, the uncertainties in th e source size and the dust opacity law at long wavelengths limit the a ccuracy with which the dominant cool dust components can be described.