Witnessing the Gradual Slow-Down of Powerful Extragalactic Jets: The X-ray - Radio Connection

Radio jets emanating from the central engine of powerful active galaxies start with relativistic velocities, as indicated by the superluminal velocities observed in their inner (pc) part. However, it is still not clear if the jets remain relativistic further out, a key question for their dynamics. Recently, it was shown that the Chandra-detected X-ray emission from knots in large scale (∼ hundreds of kpc) powerful jets is best understood as Compton scattering off the cosmic microwave background from jets that flow relativistically (Lorentz factor ∼ 10--20). A puzzling feature of this X-ray emission is that it peaks in the inner part of the jet, decreasing outward, a behavior markedly opposite to that of the radio emission. Here we show that, this naturally results form a gradual deceleration of the jet flow, and that the offsets between different wavelength maps can reveal the deceleration profile. This matches another recent finding, namely that the jet flow feeding the hot spots - bright regions where the jets terminate colliding with the intergalactic medium - is still relativistic with a Lorentz factor of ∼ a few.