The HST view of FR I radio galaxies: evidence for non-thermal nuclear sources

Unresolved nuclear sources are detected by the Hubble Space Telescope in th e great majority of a complete sample of 33 FR I radio galaxies belonging t o the 3CR catalogue. The optical flux of these Central Compact Cores (CCC) shows a striking line ar correlation with the radio core one over four decades, arguing for a non -thermal synchrotron origin of the CCC radiation. We also find evidence tha t this emission is anisotropic, which leads us to identify CCCs with the mi soriented relativistic jet component which dominates in BL Lac objects. Thi s interpretation is also supported by the similarity in the radio-to-optica l and optical spectral indices. The high rate of CCC detection (85%) suggests that a "standard" pc scale, g eometrically thick torus is not present in low luminosity radio-galaxies. T hus the lack of broad lines in FR I cannot be attributed to obscuration. CCC fluxes also represent upper limits to any thermal/disc emission. For a 10(9)M(.) black hole, typical of FR I sources, these limits translate into a fraction as small as less than or similar to 10(-7) - 10(-5) of the Eddin gton luminosity.