The Ionization of the Emission Line Gas in Nearby Powerful Radio Galaxies

TL;DR

This paper reviews the physical mechanisms ionizing emission line gas in powerful radio galaxies, emphasizing the roles of jet-induced shocks and AGN photoionization, with recent evidence favoring shocks along radio axes.

Contribution

It synthesizes morphological, kinematical, and diagnostic diagram data to clarify the dominant ionization mechanisms in extended emission line regions of radio galaxies.

Findings

Jet-induced shocks dominate ionization along radio axes.

AGN photoionization remains dominant in nuclear regions.

Combining multiple data types clarifies ionization mechanisms.

Abstract

In this article I review the dominant physical mechanisms in the extended emission line regions (EELR) of powerful radio galaxies at low redshifts and, in particular, the balance between jet-induced shocks and AGN photoionization. I consider the evidence for jet-induced shocks based on morphological, kinematical and ionization (diagnostic diagram) information. Although each of these three types of information separately provides some evidence for the effects of jet-induced shocks, this evidence is often ambiguous. The major advance in recent years has been to combine morphological, kinematical and diagnostic diagram approaches. In this way it has been possible to show that jet-induced shocks dominate the ionization and acceleration of the EELR along the radio axes of many powerful radio galaxies. However, AGN photoionization is likely to remain the dominant mechanism in the nuclear regions of radio galaxies in which the radio sources extend well beyond the emission line regions.