The twin-jet system in the Fanaroff-Riley II radio galaxy 3C 452. A sub-parsec-scale VLBI study

We present a comprehensive multifrequency VLBI analysis of the Fanaroff-Riley II (FRII) high-excitation radio galaxy 3C 452. Our aim is to resolve and analyze, for the first time, its twin-jet structure on sub-parsec scales. Our dataset comprises high-sensitivity array (HSA) observations at 4. 9, 8. 4, 15. 4, 23. 6, and 43. 2 GHz. Through fitting methods performed in both the visibility and the image plane, we were able to trace the jet expansion from scales of a few thousand to nearly 10⁵ Schwarzschild radii (R_ S) for both the approaching and receding jets. Additionally, we derived the core brightness temperatures and Doppler factors to constrain the orientation and intrinsic speed of the jet. Our study provides the first detailed description of the twin-jet system in 3C 452 on VLBI scales, confirming it as a rare FRII source with jets detected down to millimeter wavelengths. We resolved both the jet and counter-jet down to scales of a few thousand R_ R_ revealing a symmetric parabolically expanding structure with power-law indices of k≈ 0. 66 (jet) and k≈ 0. 47 (counter-jet). Jet-to-counter-jet intensity ratios remain nearly constant on larger scales but rise sharply within the inner 1–1. 5 mas, as revealed by the higher-frequency data, consistent with increasing jet speed. We observed low core brightness temperatures values, which are indicative of Doppler de-boosting (δ ∼ 0. 03–0. 83) due to the large viewing (þeta ≈ 70^̧irc) and/or a magnetically dominated jet base. A spectral index analysis revealed a strongly inverted core spectrum (α > 2) with additional absorption at the highest frequencies, followed by a sharp steepening (α∼-2. 5) to optically thin values in the innermost jet. Finally, a comparison between broad- and narrow-line high-excitation radio galaxies showed that jets in narrow-line sources, such as 3C 452 and Cygnus A, complete collimation at łesssim 10^ 5 S, whereas broad-line sources exhibit shape transitions at 10⁶–10^ 7 R_ S, suggesting that orientation plays an important role in the observed collimation scales.