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

Context. The Sloan Digital Sky Survey (SDSS) allows us to classify galaxies using optical low-ionization emission-line diagnostic diagrams. A cross-correlation of the SDSS data release 7 (DR7), containing spectroscopic data, with the Very Large Array (VLA) survey Faint Images of the Radio Sky at Twenty-centimeters (FIRST), makes it possible to conduct a joined multiwavelength statistical study of radio-optical galaxy properties on a very large number of sources. Aims. Our goal is to improve the study of the combined radio-optical data by investigating whether 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 search for a trend with z in the classification provided by classical and more recent optical emission-line diagnostic diagrams. Methods. We cross-matched the optical sources with the FIRST radio survey in order to obtain spectroscopic information of a selected sample of radio emitters with optical observed counterpart. We searched for an L-20 (cm)/L-H alpha threshold value above which the radio emitters start being classified as active galactic nuclei (AGNs) rather than star-forming galaxies (SFGs). We investigated the origin of emission-lines by using both photoionization and shock models. Results. The percentage of detected AGNs (Seyferts and LINERs) or composites is much higher in the optical-radio sample than in the optical sample alone. We find a progressive shift in the sources towards the AGN region of the diagram with increasing L-20 cm/L-H alpha, with an indication of different behavior for LINERs and Seyferts. The classification appears to slightly depend on the redshift. The diagnostic diagrams display a density peak in the star-forming or composite region for log (L-20 (cm)/L-H alpha) < 0.716, while the distribution in the LINER region peaks above this threshold. 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. Conclusions. Our results indicate that it is worthwhile to further explore the radio domain, probing the physical nature of LINERs, thanks to a combination of optical and radio information. The [N II]/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.