Shock-ionization in the Extended Emission-Line Region of 3C~305. The last piece of the (optical) puzzle

TL;DR

This study uses Gemini spectroscopic data to determine that shock ionization, rather than pure shock models, best explains the emission-line region in the radio galaxy 3C 305, revealing complex interactions between jets and ambient medium.

Contribution

It provides the first comprehensive spectroscopic analysis confirming shock ionization as the main mechanism in 3C 305's emission-line region, emphasizing auto-ionizing shock models.

Findings

Emission line ratios indicate shock ionization.

Presence of neutral outflow supports shock cooling.

Observed emission is lower than pure shock predictions.

Abstract

We present new Gemini spectroscopical data of the Extended Emission-Line Region of 3C~305 radio galaxy in order to achieve the final answer of the long-standing question about the ionizing mechanism. The spectra show strong kinematic disturbances within the most intense line-emitting region. The relative intensities amongst the emission lines agree with the gas being shocked during the interaction of the powerful radio jets with the ambient medium. The emission from the recombination region acts as a very effective cooling mechanism, which is supported by the presence of a neutral outflow. However, the observed intensity is almost an order of magnitude lower than expected in a pure shock model. So auto-ionizing shock models, in low-density and low-abundance regime, are required in order to account for the observed emission within the region. This scenario also supports the hypothesis that the optical emitting gas and the X-ray plasma are in pressure balance.