Classifying Radio Emitters from the Sloan Digital Sky Survey: Spectroscopy and Diagnostics

TL;DR

This study uses SDSS and FIRST survey data to analyze the relationship between radio and optical galaxy properties, revealing correlations that aid in classifying galaxy types and understanding their emission mechanisms.

Contribution

It introduces a large-scale multiwavelength analysis linking radio luminosity ratios with optical spectroscopic classifications, highlighting the prevalence of AGNs and LINERs and their emission origins.

Findings

Higher detection of AGNs and composites in radio-optical sample.

Correlation between radio-to-optical luminosity ratio and galaxy classification.

LINER emission lines may be explained by shocks.

Abstract

A cross-correlation of the SDSS DR7 with the FIRST radio survey makes it possible to conduct a joined multiwavelength statistical study of radio-optical galaxy properties on a very large number of sources. Our goal is to improve the study of the combined radio-optical data by investigating if there is a correlation between the radio luminosity at 20 cm over the luminosity of the optical H\alpha line (L[20 cm]/L[H\alpha]) and line excitation ratios, where the latter provide the spectroscopic classification in Seyferts, low-ionization nuclear emission-line regions (LINERs) and star-forming galaxies. We found that the percentage of detected AGNs (Seyferts and LINERs) or composites is much higher in the optical-radio sample than in the optical sample alone. There is a progressive shift of the sources towards the AGN region of the diagram with increasing L[20 cm]/L[H\alpha], with an indication for a different behavior for LINERs and Seyferts. The classification appears to slightly depend on the redshift. A comparison with photoionization and shock models shows that the large fraction of LINERs identified in our study have emission lines that may be explained by shocks. The [NII]/H\alpha vs. equivalent width of the H\alpha line (WHAN) diagram confirms the LINER classification for most of those that have been identified with the traditional diagnostic diagrams. The correlation between L[20 cm]/L[H\alpha] and optical emission line ratios suggests the nuclear origin of the emission from the most powerful radio-galaxies.