  We present the first collective evidence that Fermi-detected jets of high
kinetic power (L_kin) are dominated by inverse Compton emission from
upscattered external photons. Using a sample with a broad range in orientation
angle, including radio galaxies and blazars, we find that very high power
sources (L_kin > 10^45.5 erg s^{-1}) show a significant increase in the ratio
of inverse Compton to synchrotron power (Compton dominance) with decreasing
orientation angle, as measured by the radio core dominance and confirmed by the
distribution of superluminal speeds. This increase is consistent with beaming
expectations for external Compton (EC) emission, but not for synchrotron self
Compton (SSC) emission. For the lowest power jets (L_kin < 10^43.5 erg s^{-1}),
no trend between Compton and radio core dominance is found, consistent with
SSC. Importantly, the EC trend is not seen for moderately high power flat
spectrum radio quasars with strong external photon fields. Coupled with the
evidence that jet power is linked to the jet speed (Kharb et al. 2010), this
finding suggests that external photon fields become the dominant source of seed
photons in the jet comoving frame only for the faster and therefore more
powerful jets.
