A Statistical Study of Emission Lines from High Redshift Radio Galaxies

TL;DR

This study analyzes emission line properties in 165 high-redshift radio galaxies, revealing correlations between line features, radio parameters, and ionization mechanisms, suggesting a combined shock and photo-ionization model fits the data best.

Contribution

It provides a comprehensive statistical analysis of emission lines and radio properties in high-redshift radio galaxies, highlighting the role of shock and photo-ionization in their emission regions.

Findings

Significant correlations between Ly-alpha asymmetry and radio size/redshift.

Line luminosities correlate with radio power.

A combined shock and photo-ionization model best explains the emission spectra.

Abstract

We have compiled a sample of 165 radio galaxies from the literature to study the properties of the extended emission line regions and their interaction with the radio source over a large range of redshift 0<z<5.2. For each source, we have collected radio (size, lobe distance ratio and power) and spectroscopic parameters (luminosity, line width and equivalent width) for the four brightest UV lines. We also introduce a parameter A_{Ly-alpha} measuring the asymmetry of the Ly-alpha line. Using these 18 parameters, we examine the statistical significance of all 153 mutual correlations, and find the following significant correlations: (i) Ly-alpha asymmetry A_{Ly-alpha} with radio size and redshift, (ii) line luminosity with radio power, (iii) line luminosities of UV lines with each other, and (iv) equivalent widths of UV lines with each other. Using line-ratio diagnostic diagrams, we examine the ionization mechanism of the extended emission line regions in HzRGs. The high ionization lines seem to confirm previous results showing that AGN photo-ionization provides the best fit to the data, but are inconsitent with the CII/CIII ratio, which favour the highest velocity shock ionization models. We note that the CII line is 5 times more sensitive to shock ionization than the high ionization UV lines, and show that a combination of shock and photo-ionization provides a better overall fit to the integrated spectra of HzRGs. Because most HzRGs have radio sizes <~150 kpc, their integrated spectra might well contain a significant contribution from shock ionized emission. [abridged]