  We have performed infrared imaging of the jet of the quasar 3C 273 at
wavelengths 3.6 and 5.8 microns with the Infrared Array Camera (IRAC) on the
Spitzer Space Telescope. When combined with the radio, optical and X-ray
measurements, the IRAC photometry clearly shows that the optical emission is
dominated by the high-energy component of the jet, not by the radio synchrotron
component, as had been assumed to date. The high-energy component may be due to
a second synchrotron component or to IC scattering of ambient photons. In the
former case, we argue that the acceleration of protons exceeding 10^16 eV or
possibly even to 10^19 eV would be taking place in the jet. In contrast, the IC
model, into which highly relativistic Doppler beaming has to be incorporated,
requires very low-energy electrons (~ 1 MeV). The present polarization data in
the radio and optical would favor the former interpretation in the case of the
3C 273 jet. Sensitive and detailed measurements of optical polarization are
important to establish the radiation mechanism responsible for the high-energy
emission. The present study offers new clues as to the controversial origin of
the X-ray emission seen in many quasar jets.
