  Several BL Lac objects are confirmed sources of variable and strongly
Doppler-boosted TeV emission produced in the nuclear portions of their
relativistic jets. It is more than probable, that also many of the FR I radio
galaxies, believed to be the parent population of BL Lacs, are TeV sources, for
which Doppler-hidden nuclear gamma-ray radiation may be only too weak to be
directly observed. Here we show, however, that about one percent of the total
time-averaged TeV radiation produced by the active nuclei of low-power FR I
radio sources is inevitably absorbed and re-processed by photon-photon
annihilation on the starlight photon field, and the following emission of the
created and quickly isotropized electron-positron pairs. In the case of the
radio galaxy Centaurus A, we found that the discussed mechanism can give a
distinctive observable feature in the form of an isotropic gamma-ray halo. It
results from the electron-positron pairs injected to the interstellar medium of
the inner parts of the elliptical host by the absorption process, and
upscattering starlight radiation via the inverse-Compton process mainly to the
GeV-TeV photon energy range. Such a galactic gamma-ray halo is expected to
possess a characteristic spectrum peaking at ~0.1 TeV photon energies, and the
photon flux strong enough to be detected by modern Cherenkov Telescopes and, in
the future, by GLAST. These findings should apply as well to the other nearby
FR I sources.
