Abstract
We address the question of why low-luminosity radio sources with similar flat radio spectra show a range of optical activity. The investigation is based on the spectral energy distributions (SEDs) of objects from the 200 mJy sample. We gathered new data from the VLA at 43 GHz, from SCUBA in the JCMT at 2000, 1350 and 850 micron and from the ISOPHOT instrument on ISO at 170, 90, 60 and 25 micron. There is considerable diversity amongst the SEDs of the objects: there are objects with steep broadband spectra between centimetre and millimetre bands (14% of the sample), there are those with flat broadband spectra over most of the spectral range (48% of the sample) and there are those which show pronounced sub-mm/infrared excesses (27% of the sample). Some objects of the first group have two sided radio morphology indicating that their pc-scale emission is not dominated by beamed jet emission. Amongst the objects that have smooth broadband spectra from the radio to the infrared there are passive elliptical galaxies as well as the expected BL Lacs objects. The most pronounced sub-mm/infrared excesses are shown by the broad emission line objects.
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