  We report the Fermi Large Area Telescope detection of extended gamma-ray
emission from the lobes of the radio galaxy Fornax A using 6.1 years of Pass 8
data. After Centaurus A, this is now the second example of an extended
gamma-ray source attributed to a radio galaxy. Both an extended flat disk
morphology and a morphology following the extended radio lobes were preferred
over a point-source description, and the core contribution was constrained to
be < 14% of the total gamma-ray flux. A preferred alignment of the gamma-ray
elongation with the radio lobes was demonstrated by rotating the radio lobes
template. We found no significant evidence for variability on ~0.5 year
timescales. Taken together, these results strongly suggest a lobe origin for
the gamma rays. With the extended nature of the > 100 MeV gamma-ray emission
established, we model the source broadband emission considering currently
available total lobe radio and millimeter flux measurements, as well as X-ray
detections attributed to inverse Compton (IC) emission off the cosmic microwave
background (CMB). Unlike the Centaurus A case, we find that a leptonic model
involving IC scattering of CMB and extragalactic background light (EBL) photons
underpredicts the gamma-ray fluxes by factors of about ~ 2 - 3, depending on
the EBL model adopted. An additional gamma-ray spectral component is thus
required, and could be due to hadronic emission arising from proton-proton
collisions of cosmic rays with thermal plasma within the radio lobes.
