Optical synchrotron emission from the nuclei of FR I radio galaxies

Abstract

We studied HST images of a complete sample of 33 FR I radio galaxies. Emission from an unresolved Central Compact Core (CCC) is detected in the great majority of the objects. CCC optical emission shows a striking correlation with the radio core fluxes, which extends over four decades with a slope of ∼1. This argues strongly for a non-thermal synchrotron origin of the CCCs, which would then dominate the nuclear emission both in the radio and in the optical bands. The high detection rate of CCCs sets strong constraints on the existence/geometry of any obscuring, optically thick structure (torus) in FR I. We also find evidence for a different behavior of low and high power objects belonging to our sample. CCC fluxes also represent upper limits to any unobscured thermal/disc emission, which translate into a fraction as small as ≲10 −7−10 −5 of the Eddington luminosity (for a 10 9 M ⊙ black hole).