The submillimeter spectral break in Seyfert galaxies

Abstract

We have observed four Seyfert 1 galaxies at 155 microns and three at 370 microns. Two of the galaxies were detected at 155 microns, but none was detected at 370 microns. The 155 micron data and the strong upper limits at 370 microns show that the infrared continua decrease sharply beyond 100-155 microns. These observations depict a clear spectral difference between radio-loud and radio-quiet active galactic nuclei. The limits on the 100-370 micron spectral slope are consistent either with synchrotron self-absorption of a nonthermal source or with thermal dust emission similar to that arising from the disks of normal galaxies. The 155 micron flux from NGC 4151 is spatially extended through a radius of at least 48", arguing for substantial dust emission at λ > 80 microns and an even shorter cutoff wavelength for nonthermal emission from the active nucleus. The spectral breaks implied by our data suggest that the nuclear nonthermal sources must be smaller than ~10 light hours.