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

TL;DR

This study measures proper motions in the jet of galaxy 3C 78, revealing jet acceleration from parsec to kiloparsec scales and providing new constraints on jet plasma physics and dynamics.

Contribution

It offers the first direct evidence of jet acceleration in a conical, matter-dominated AGN jet using multi-scale proper motion measurements.

Findings

Knot B moves at ~0.6c, indicating subluminal motion.

Knot C shows apparent backward motion at ~-2.6c, then forward at ~0.5c.

Jet undergoes bulk acceleration from parsec to kiloparsec scales.

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 ($\sim0.6c$ at $\sim$ 200 pc), while knot C undergoes extreme (apparent) backward motion and eventual forward motion ($\sim-2.6c$, $0.5c$, at $\sim$ 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 ($\sim0.1c$) 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.