The spectral energy distributions of the revised 200-mJy sample.

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 μm, and from the ISOPHOT instrument on ISO at 170, 90, 60 and 25 μm. There is considerable diversity amongst the SEDs of the objects: there are objects with steep broad-band spectra between centimetre and millimetre bands (14 per cent of the sample); there are those with flat broad-band spectra over most of the spectral range (48 per cent of the sample); and there are those which show pronounced submillimetre/infrared excesses (27 per cent of the sample). Some objects of the first group have two-sided radio morphology, indicating that their parsec-scale emission is not dominated by beamed jet emission. Amongst the objects that have smooth broad-band spectra from the radio to the infrared, there are passive elliptical galaxies as well as the expected BL Lacertae objects. The most pronounced submillimetre/infrared excesses are shown by the broad-emission-line objects. [ABSTRACT FROM AUTHOR]