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 the great majority of a complete sample of 33 FR I radio galaxies belonging to the 3CR catalogue. The optical flux of these Central Compact Cores (CCC) shows a striking linear correlation with the radio core one over four decades, arguing for a non-thermal synchrotron origin of the CCC radiation. We also find evidence that this emission is anisotropic, which leads us to identify CCCs with the misoriented relativistic jet component which dominates in BL Lac objects. This interpretation is also supported by the similarity in the radio-to-optical and optical spectral indices. The high rate of CCC detection (85%) suggests that a ``standard'' pc scale, geometrically thick torus is not present in low luminosity radio-galaxies. Thus 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 <~ 10^{-7}-10^{-5} of the Eddington luminosity. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555 and by STScI grant GO-3594.01-91A