CAgNVAS II. Proper Motions in the sub-kiloparsec Jet of 3C 78: Novel Constraints on the Physical Nature of Relativistic Jets

Abstract

Jets from active galactic nuclei are thought to play a role in the evolution of their host and local environments, but a detailed prescription is limited by the understanding of the jets themselves. Proper motion studies of compact bright components in radio jets can be used to produce model-independent constraints on their Lorentz factor, necessary to understand the quantity of energy deposited in the inter-galactic medium. We present our initial work on the jet of radio-galaxy 3C~78, as part of CAgNVAS (Catalogue of proper motions in Active galactic Nuclei using Very Large Array Studies), with a goal of constraining nature of jet plasma on larger (>100 parsec) scales. In 3C~78 we find three prominent knots (A, B and C), where knot B undergoes subluminal longitudinal motion (∼0.6c at ∼ 200 pc), while knot C undergoes extreme (apparent) backward motion and eventual forward motion (∼−2.6c, 0.5c, at ∼ 300 pc). Assuming knots are shocks, we infer the bulk speeds from the pattern motion of Knots B and C. We model the spectral energy distribution (SED) of the large-scale jet and observe that a physically motivated two-zone model can explain most of the observed emission. We also find that the jet profile remains approximately conical from parsec to kiloparsec scales. Using the parsec-scale speed from VLBI studies (∼0.1c) and the derived bulk speeds, we find that the jet undergoes bulk acceleration between the parsec and the kiloparsec scales providing the first direct evidence of jet acceleration in a conical and matter-dominated jet.