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

Measurements of 11 hard X-ray selected active galaxies at 800 and 1100 microns made with the James Clerk Maxwell Telescope are reported, and these are discussed 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 nonthermal origin. Any underlying synchrotron components must become self-absorbed by a few tens of microns, implying that such nonthermal 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. The alternative possibility that IR emission is entirely due to thermal dust reemission of the observed UV continuum is examined. Using a physical model in which the radiative transfer through a spherical dust cloud is solved, it is found that to explain both the continuum shape over 5 microns to 1 mm, and the lack of silicate absorption, requires a region with tau (UV) of about 10, and with density following sq rt 1.