Searching for evidence of jet-cloud interaction in radio galaxies. First results for 3C 381

TL;DR

This study investigates the ionisation mechanisms in the radio galaxy 3C 381, finding that shock-ionisation models better explain the observed emission lines and velocity features than photoionisation models.

Contribution

It provides new evidence supporting shock-ionisation as the dominant mechanism in 3C 381's emission-line regions, challenging previous photoionisation assumptions.

Findings

Shock-ionisation models fit high-excitation lines better.

Shocks with velocities >500 km/s explain line ratios.

Mechanical energy from shocks accounts for high-velocity line-splitting.

Abstract

We present results of Gemini spectroscopy and Hubble Space Telescope imaging of the 3C~381 radio galaxy. Possible ionising mechanisms for the Extended Emission-Line Region were studied through state-of-the-art diagnostic analysis employing line-ratios. Photoionisation from the central engine as well as mixed-medium photoionisation models fail in reproducing both the strengths and the behaviour of the highest-excitation lines, such as [NeV]3424, HeII, and [OIII}]5007, which are measured at very large distances from the AGN. Shock-ionisation models provide a better fit to the observation. Expanding shocks with velocities higher than 500 km/s are capable of reaching the observed intensity ratios for lines with different ionisation states and excitation degrees. This model also provide a direct explanation of the mechanical energy input needed to explain the high-velocity line-splitting observed in the velocity field.