The fingerprints of Photoionization and Shock-Ionization in two CSS sources

TL;DR

This study analyzes the ionization mechanisms in the extended emission-line regions of two CSS radio galaxies, demonstrating that shock-ionization combined with AGN photoionization explains observed spectral features better than pure photoionization models.

Contribution

It provides new spectroscopic evidence showing the combined role of shock-ionization and photoionization in CSS sources, especially highlighting the significance of [Ne V] emission.

Findings

[Ne V] emission challenges pure photoionization models.

Shock-ionization models better explain line ratios and spatial extension.

External ionizing fields, like shocks or AGN radiation, are influential.

Abstract

We investigate the ionization state of the Extended Emission-Line Regions (EELRs) around two compact steep-spectrum (CSS) radio galaxies, 3C~268.3 and 3C~303.1, in order to identify the contribution of photoionization and shock-ionization. We perform a new spectroscopical (long-slit) analysis with GMOS/Gemini with the slit oriented in the radio-jet direction, where outflows are known to exist. The [Ne V]$\lambda 3426$ emission is the most interesting feature of the spectra and the key to breaking the degeneracy between the models: since this emission-line is more extended than HeII, it challenges the ionization structure proposed by any photoionization model, also its intensity relative to H$\beta$ does not behave as expected with respect to the ionization parameter U in the same scenario. On the contrary, when it is compared to the intensity of [OII]$\lambda3727$/H$\beta$ and all these results are joined, the whole scenario is plausible to be explained as emission coming from the hot, compressed, shocked gas in shock-ionization models. Although the model fitting is strongly sensitive to the chosen line-ratios, it argues for the presence of external and strong ionizing fields, such as the precursor field created by the shock or/and the AGN radiation field. In this paper, we show how AGN photoionization and shock-ionization triggered by jet-cloud interaction work together in these EELRs in order to explain the observed trends and line-ratio behaviours in a kinematically acceptable way.