We report submillimeter continuum observations of six radio-quiet active galactic nuclei selected for high IRAS flux, in order to test models for the infrared continuum by constraining the far- infrared/submillimeter cutoff slope. Slopes greater than the self- absorbed synchrotron cutoff α_cutoff_ = 2.5 (F_V_ is proportional to Valpha^) argue against the synchrotron model and lend support to the dust model. (The synchrotron cutoff slope can be slightly sharper than 2.5 if concave electron energy distributions and extreme homogeneity are assumed.) We detected the luminous Seyfert 1 galaxy PG 1434+590 at 438 and 345 microns; combining these detections with the IRAS 100 micron flux gives a slope of 2.53 +/- 0.17. Using our observed submillimeter upper limits for the radio-quiet quasars, we were able to constrain the cutoff slopes such that α_cutoff_ > 2.50 in two cases (data for three others do not provide significant slope constraints). We conclude that the dust model is favored, but that synchrotron emission cannot yet be ruled out conclusively. In a related program we also observed and detected the z = 2.546 gravitationally lensed quasar H1413+ 117, the "cloverleaf", at 345,438, and 761 microns. The slope between 438 and 761 microns (observed; 124-215 micron rest frame) in this object is found to be 2.95 +/- 0.69. We Suggest that in H1413 + 117 we may be seeing a highly luminous but otherwise rather normal far-infrared peak shifted well into the submillimeter region
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Barvainis et al. (1992) studied this question.