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