Millimetre measurements of hard X-ray selected active galaxies: implications for the nature of the continuous spectrum

We report measurements of 11 hard X-ray selected active galaxies at 800 and 1100 μm made with the James Clerk Maxwell Telescope, and discuss these in the context of the continuum energy distribution from radio to X-ray wavelengths. Four other radio-loud AGN were also measured. Radio-loud objects show a spectrum which decreases smoothly in flux to higher frequencies, and supporting evidence strongly suggests a non-thermal origin. For radio-quiet objects we report only upper limits, but in all cases the fall from 100 μm to 1 mm is steep, strongly suggesting that thermal emission dominates the far-IR emission. Any underlying synchrotron components must become self-absorbed by a few tens of μm, implying that such non-thermal sources would have sizes of the order light hours (in conflict with the lack of far-IR variability) and should be heavily dominated by Compton scattering. We examine the alternative possibility that IR emission is entirely due to thermal dust re-emission of the observed UV continuum. Using a physical model in which we solve the radiative transfer through a spherical dust cloud, we find that to explain both the continuum shape over 5 μm to 1 mm, and the lack of silicate absorption, we require a region with τuv ∼ 10, and with density following r-1. To explain the continuum near 1 μm seems to need a strong stellar component, even in the unresolved nucleus, but we argue that such a nuclear star cluster may in fact be present. The strongest objections to such thermal models are the unexplained fine-tuning required, and a possible energy balance problem.