Shedding New Light on the 3C 273 Jet with theSpitzer Space Telescope

We have performed infrared imaging of the jet of the quasar 3C 273 atwavelengths 3.6 and 5.8 microns with the Infrared Array Camera (IRAC) on theSpitzer Space Telescope. When combined with the radio, optical and X-raymeasurements, the IRAC photometry clearly shows that the optical emission isdominated by the high-energy component of the jet, not by the radio synchrotroncomponent, as had been assumed to date. The high-energy component may be due toa second synchrotron component or to IC scattering of ambient photons. In theformer case, we argue that the acceleration of protons exceeding 10^16 eV orpossibly even to 10^19 eV would be taking place in the jet. In contrast, the ICmodel, into which highly relativistic Doppler beaming has to be incorporated,requires very low-energy electrons (~ 1 MeV). The present polarization data inthe radio and optical would favor the former interpretation in the case of the3C 273 jet. Sensitive and detailed measurements of optical polarization areimportant to establish the radiation mechanism responsible for the high-energyemission. The present study offers new clues as to the controversial origin ofthe X-ray emission seen in many quasar jets.