2018ApJ...854..148N

We report new measurements of jet width and radiation profiles along jets over the range of 10³-10⁹ Schwarzschild radius (R_S) in the nearby radio galaxy NGC 4261 using multifrequency Very Long Baseline Array (VLBA) and Very Large Array images. In the VLBA images, we found parabolic-to-conical transition signatures on both the approaching jet and counterjet width profiles at ∼10⁴ R_S from the central engine. A transition in the radiation profile along the approaching jet was also found at ∼10⁴ R_S clearly. Based on the consistency of the transition locations, we conclude that the physical conditions of the NGC 4261 jets change at this distance. Jet flows that change from accelerating to expanding regions were previously found in M87 and NGC 6251 and are presumably present in NGC 4261. Additionally, we found another transition in the radiation profile at ∼3 x 10⁶ R_S in the conical region. NGC 4261 is the first case in which a jet structural transition is suggested in both the approaching and counter jets; this implies that the active galactic nucleus jet collimation process is fundamentally characterized by the global distribution of ambient pressure rather than the local interaction between the jet and the surrounding medium. We discuss the evolution of jet conditions in terms particle acceleration, cooling, dissipation, and jet pressure balance with the surrounding hot gas.