The Origin of High Energy Emission in the Young Radio Source PKS
  1718-649
  We present a model for the broadband radio-to-$\gamma$-ray spectral energy
distribution of the compact radio source, PKS 1718-649. Because of its young
age (100 years) and proximity ($z=0.014$), PKS 1718-649 offers a unique
opportunity to study nuclear conditions and the jet/host galaxy feedback
process at the time of an initial radio jet expansion. PKS 1718-649 is one of a
handful of young radio jets with $\gamma$-ray emission confirmed with the
Fermi/LAT detector. We show that this $\gamma$-ray emission can be successfully
explained by Inverse Compton scattering of the ultraviolet photons, presumably
from an accretion flow, off non-thermal electrons in the expanding radio lobes.
The origin of the X-ray emission in PKS 1718-649 is more elusive. While Inverse
Compton scattering of the infrared photons emitted by a cold gas in the
vicinity of the expanding radio lobes contributes significantly to the X-ray
band, the data require that an additional X-ray emission mechanism is at work,
e.g. a weak X-ray corona or a radiatively inefficient accretion flow, expected
from a LINER type nucleus such as that of PKS 1718-649. We find that the jet in
PKS 1718-649 has low power, $L_j \simeq 2.2 \times 10^{42}$ erg s$^{-1}$, and
expands in an environment with density $n_0 \simeq 20$ cm$^{-3}$. The inferred
mass accretion rate and gas mass reservoir within 50-100 pc are consistent with
estimates from the literature obtained by tracing molecular gas in the
innermost region of the host galaxy with SINFONI and ALMA.
