The elusive optical jets

Imaging observations in the U band of eight radio galaxies are presented. No optical counter-part to the radio jets are found. For the three radio galaxies 3C 147, 3C 279, and 3C 433, it is shown that the radio to optical spectral index of the jet is significantly higher than the typical values found in the three best-known opticals jets (M87, 3C 273, and 3C 66B). It is concluded that the cutoff frequencies are lower than 10 to the 14th Hz in these cases. For the 3C 31 jet, data are consistent with the radio to optical spectral index being comparable to the typical values. This result is in contradiction with the detection of the optical jet in the B band by Butcher et al. (1980). Finally, the lower limit on the radio to optical spectral index obtained for the four other radio jets of the sample is still consistent with the typical values. (Author)