Evidence from sub-millimetre observations for thermal dust emission in NGC4151

Most quasars and Seyfert 1 galaxies have spectra that rise from optical wavelengths into the 60-100 μm infrared region, beyond which they must suffer a sharp cutoff indicated by the lack of observed millimetre emission. There is evidence^1-4 that the infrared emission receives contributions from both thermal emission by interstellar dust and synchrotron radiation from a compact source. Determining the nature of the infrared emission is thus an important step in understanding the structure of quasars and Seyfert 1 galaxies. We report here observations at 438-μm wavelength of the Seyfert 1 galaxy NGC4151 using the UKT14 bolometer on the James Clerk Maxwell Telescope. We find a 5σ upper limit of 200 mJy in an 11″ aperture. Comparison with an earlier measurement at 155-μm indicates that at least half of the 155-μm flux is due to thermal dust emission. The remainder may be from a synchrotron source which becomes self-absorbed at wavelengths of less than 80 μm.