2005ApJ...631L.113U

Emission from the relativistic jet located hundreds of kiloparsecs from the core of the superluminal quasar PKS 0637-752 was detected at 3.6 and 5.8 µm with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. The unprecedented sensitivity and arcsecond resolution of IRAC allows us to explore the mid-infrared emission from kiloparsec-scale quasar jets for the first time. The mid-infrared flux from the jet knots, when combined with radio and optical fluxes, confirms the synchrotron origin of the radio-to-optical emission and constrains very well the high-energy end of the nonthermal electron distribution. Assuming that the X-rays are produced in the relativistically moving knots via inverse Compton scattering of cosmic microwave background (CMB) radiation, the infrared observation puts constraints on the matter content of the large-scale quasar jet. Specifically, pure e⁺e^– pair jet models are unfavorable based on the lack of an infrared bump associated with the ``bulk Comptonization'' of CMB photons by an ultrarelativistic jet.